diff --git a/web-player/package-lock.json b/web-player/package-lock.json
index 1e0fddf..377c7e2 100644
--- a/web-player/package-lock.json
+++ b/web-player/package-lock.json
@@ -36,41 +36,8 @@
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+				"vite": "^8.0.16",
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 		"node_modules/@emnapi/wasi-threads": {
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+			"resolved": "https://registry.npmjs.org/@jridgewell/sourcemap-codec/-/sourcemap-codec-1.6.0.tgz",
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 			"license": "MIT",
+			"peer": true,
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 			"license": "MIT",
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@@ -1565,12 +1513,30 @@
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+			"license": "MIT",
+			"dependencies": {
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+				"assertion-error": "^2.0.1"
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 			"license": "MIT"
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+		"node_modules/@types/deep-eql": {
+			"version": "4.0.2",
+			"resolved": "https://registry.npmjs.org/@types/deep-eql/-/deep-eql-4.0.2.tgz",
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+			"dev": true,
+			"license": "MIT"
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 			"devOptional": true,
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@@ -1843,11 +1812,70 @@
 				"url": "https://opencollective.com/typescript-eslint"
 			}
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+		"node_modules/@vitest/mocker": {
+			"version": "5.0.1",
+			"resolved": "https://registry.npmjs.org/@vitest/mocker/-/mocker-5.0.1.tgz",
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+			"license": "MIT",
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+				"@vitest/spy": "5.0.1",
+				"estree-walker": "^3.0.3",
+				"magic-string": "^1.2.3"
+			},
+			"funding": {
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+			},
+			"peerDependencies": {
+				"msw": "^2.4.9",
+				"vite": "^6.0.0 || ^7.0.0 || ^8.0.0"
+			},
+			"peerDependenciesMeta": {
+				"msw": {
+					"optional": true
+				},
+				"vite": {
+					"optional": true
+				}
+			}
+		},
+		"node_modules/@vitest/mocker/node_modules/estree-walker": {
+			"version": "3.0.3",
+			"resolved": "https://registry.npmjs.org/estree-walker/-/estree-walker-3.0.3.tgz",
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+			"license": "MIT",
+			"dependencies": {
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+			"license": "MIT",
+			"dependencies": {
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+			}
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+		"node_modules/@vitest/spy": {
+			"version": "5.0.1",
+			"resolved": "https://registry.npmjs.org/@vitest/spy/-/spy-5.0.1.tgz",
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+			"dev": true,
+			"license": "MIT",
+			"funding": {
+				"url": "https://opencollective.com/vitest"
+			}
+		},
 		"node_modules/acorn": {
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 			"license": "MIT",
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@@ -1891,6 +1919,16 @@
 				"node": ">= 0.4"
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+			"resolved": "https://registry.npmjs.org/assertion-error/-/assertion-error-2.0.1.tgz",
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+			"dev": true,
+			"license": "MIT",
+			"engines": {
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@@ -1923,6 +1961,16 @@
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+			"resolved": "https://registry.npmjs.org/chai/-/chai-6.2.2.tgz",
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+			"dev": true,
+			"license": "MIT",
+			"engines": {
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+			}
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@@ -2063,6 +2111,13 @@
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+			"resolved": "https://registry.npmjs.org/es-module-lexer/-/es-module-lexer-2.3.2.tgz",
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+			"license": "MIT"
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@@ -2323,6 +2379,16 @@
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+			"license": "Apache-2.0",
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@@ -3160,6 +3226,7 @@
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@@ -3573,6 +3643,13 @@
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+			"license": "ISC"
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@@ -3596,6 +3673,20 @@
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+			"license": "MIT"
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@@ -3760,6 +3852,26 @@
 				"url": "https://opencollective.com/webpack"
 			}
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+		"node_modules/tinybench": {
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+			"license": "MIT",
+			"engines": {
+				"node": ">=20.0.0"
+			}
+		},
+		"node_modules/tinyexec": {
+			"version": "1.3.0",
+			"resolved": "https://registry.npmjs.org/tinyexec/-/tinyexec-1.3.0.tgz",
+			"integrity": "sha512-QKAl9m8gWWGHV8jZcPeym6j+XULi6tOf1mT83WYJ4Lk2ytW/uwAWkrP0uFsdoYMdueVJ0qs26wZ+23xeB4ibNQ==",
+			"dev": true,
+			"license": "MIT",
+			"engines": {
+				"node": ">=18"
+			}
+		},
 		"node_modules/tinyglobby": {
 			"version": "0.2.17",
 			"resolved": "https://registry.npmjs.org/tinyglobby/-/tinyglobby-0.2.17.tgz",
@@ -3824,6 +3936,7 @@
 			"integrity": "sha512-y2TvuxSZPDyQakkFRPZHKFm+KKVqIisdg9/CZwm9ftvKXLP8NRWj38/ODjNbr43SsoXqNuAisEf1GdCxqWcdBw==",
 			"devOptional": true,
 			"license": "Apache-2.0",
+			"peer": true,
 			"bin": {
 				"tsc": "bin/tsc",
 				"tsserver": "bin/tsserver"
@@ -3885,6 +3998,7 @@
 			"resolved": "https://registry.npmjs.org/vite/-/vite-8.1.3.tgz",
 			"integrity": "sha512-Ds+gBRbj0lwRO2Y5hwnUBdxSwlAve9LeRyU4sNnAr0ewW0gWF0n5bgXgUzbgZ49MV9BVUAQUFYVcDUcilUExMA==",
 			"license": "MIT",
+			"peer": true,
 			"dependencies": {
 				"lightningcss": "^1.32.0",
 				"picomatch": "^4.0.4",
@@ -3976,6 +4090,99 @@
 				}
 			}
 		},
+		"node_modules/vitest": {
+			"version": "5.0.1",
+			"resolved": "https://registry.npmjs.org/vitest/-/vitest-5.0.1.tgz",
+			"integrity": "sha512-iA95lQbKEkvrtTkdAgnWbXfbipWiiWe/hDl2P5tMi6WFwD76G0NxXAGp/M9EOcYupeGJRr6wppMc7CoA41TQjg==",
+			"dev": true,
+			"license": "MIT",
+			"dependencies": {
+				"@types/chai": "^5.2.2",
+				"@vitest/mocker": "5.0.1",
+				"chai": "^6.2.2",
+				"es-module-lexer": "^2.3.2",
+				"expect-type": "^1.4.0",
+				"magic-string": "^1.2.3",
+				"obug": "^2.1.4",
+				"picomatch": "^4.0.7",
+				"std-env": "^4.2.0",
+				"tinybench": "6.1.4",
+				"tinyexec": "1.3.0",
+				"tinyglobby": "^0.2.17",
+				"why-is-node-running": "^2.3.0"
+			},
+			"bin": {
+				"vitest": "vitest.mjs"
+			},
+			"engines": {
+				"node": "^22.12.0 || ^24.0.0 || >=26.0.0"
+			},
+			"funding": {
+				"url": "https://opencollective.com/vitest"
+			},
+			"peerDependencies": {
+				"@edge-runtime/vm": "*",
+				"@opentelemetry/api": "^1.9.0",
+				"@types/node": "^22.0.0 || >=24.0.0",
+				"@vitest/browser-playwright": "5.0.1",
+				"@vitest/browser-preview": "5.0.1",
+				"@vitest/browser-webdriverio": "^5.0.0-beta.5 || >=5.0.0",
+				"@vitest/coverage-istanbul": "5.0.1",
+				"@vitest/coverage-v8": "5.0.1",
+				"@vitest/ui": "5.0.1",
+				"happy-dom": "*",
+				"jsdom": "*",
+				"vite": "^6.4.0 || ^7.0.0 || ^8.0.0"
+			},
+			"peerDependenciesMeta": {
+				"@edge-runtime/vm": {
+					"optional": true
+				},
+				"@opentelemetry/api": {
+					"optional": true
+				},
+				"@types/node": {
+					"optional": true
+				},
+				"@vitest/browser-playwright": {
+					"optional": true
+				},
+				"@vitest/browser-preview": {
+					"optional": true
+				},
+				"@vitest/browser-webdriverio": {
+					"optional": true
+				},
+				"@vitest/coverage-istanbul": {
+					"optional": true
+				},
+				"@vitest/coverage-v8": {
+					"optional": true
+				},
+				"@vitest/ui": {
+					"optional": true
+				},
+				"happy-dom": {
+					"optional": true
+				},
+				"jsdom": {
+					"optional": true
+				},
+				"vite": {
+					"optional": false
+				}
+			}
+		},
+		"node_modules/vitest/node_modules/magic-string": {
+			"version": "1.4.2",
+			"resolved": "https://registry.npmjs.org/magic-string/-/magic-string-1.4.2.tgz",
+			"integrity": "sha512-vG+rjFRj1PqdIBozIxAGMjPlOhaVe+GXpbttY/iSK7rGcJRMlwNJO7dcUwmUqkymsFLJiNGI06t4D7Fr7yRC9g==",
+			"dev": true,
+			"license": "MIT",
+			"dependencies": {
+				"@jridgewell/sourcemap-codec": "^1.6.0"
+			}
+		},
 		"node_modules/which": {
 			"version": "2.0.2",
 			"resolved": "https://registry.npmjs.org/which/-/which-2.0.2.tgz",
@@ -3992,6 +4199,23 @@
 				"node": ">= 8"
 			}
 		},
+		"node_modules/why-is-node-running": {
+			"version": "2.3.0",
+			"resolved": "https://registry.npmjs.org/why-is-node-running/-/why-is-node-running-2.3.0.tgz",
+			"integrity": "sha512-hUrmaWBdVDcxvYqnyh09zunKzROWjbZTiNy8dBEjkS7ehEDQibXJ7XvlmtbwuTclUiIyN+CyXQD4Vmko8fNm8w==",
+			"dev": true,
+			"license": "MIT",
+			"dependencies": {
+				"siginfo": "^2.0.0",
+				"stackback": "0.0.2"
+			},
+			"bin": {
+				"why-is-node-running": "cli.js"
+			},
+			"engines": {
+				"node": ">=8"
+			}
+		},
 		"node_modules/word-wrap": {
 			"version": "1.2.5",
 			"resolved": "https://registry.npmjs.org/word-wrap/-/word-wrap-1.2.5.tgz",
diff --git a/web-player/package.json b/web-player/package.json
index 669ebd9..7d72333 100644
--- a/web-player/package.json
+++ b/web-player/package.json
@@ -11,9 +11,11 @@
 		"check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
 		"check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch",
 		"gen:api": "npx -y openapi-typescript@7 http://localhost:5111/openapi/v1.json -o src/lib/api/schema.d.ts",
+		"test": "vitest run",
+		"test:watch": "vitest",
 		"format": "prettier --write .",
 		"lint:tokens": "node scripts/check-tokens.mjs",
-		"lint": "prettier --check . && eslint . && npm run lint:tokens"
+		"lint": "prettier --check . && eslint . && npm run lint:tokens && npm run test"
 	},
 	"devDependencies": {
 		"@eslint/js": "^10.0.1",
@@ -33,7 +35,8 @@
 		"tailwindcss": "^4.3.0",
 		"typescript": "^6.0.3",
 		"typescript-eslint": "^8.64.0",
-		"vite": "^8.0.16"
+		"vite": "^8.0.16",
+		"vitest": "^5.0.1"
 	},
 	"dependencies": {
 		"@fontsource-variable/bricolage-grotesque": "^5.3.0",
diff --git a/web-player/src/lib/data/collections.test.ts b/web-player/src/lib/data/collections.test.ts
new file mode 100644
index 0000000..ee369f6
--- /dev/null
+++ b/web-player/src/lib/data/collections.test.ts
@@ -0,0 +1,23 @@
+import { describe, expect, it } from 'vitest';
+import { appendUnique, shuffle } from './collections';
+
+describe('appendUnique', () => {
+	const keyOf = (item: { id: number }) => item.id;
+	it('appends only unseen keys', () => {
+		const result = appendUnique([{ id: 1 }], [{ id: 1 }, { id: 2 }, { id: 2 }, { id: 3 }], keyOf);
+		expect(result).toEqual([{ id: 1 }, { id: 2 }, { id: 3 }]);
+	});
+	it('returns the same array when nothing is new', () => {
+		const current = [{ id: 1 }];
+		expect(appendUnique(current, [{ id: 1 }], keyOf)).toBe(current);
+	});
+});
+
+describe('shuffle', () => {
+	it('keeps the same elements and does not mutate the input', () => {
+		const input = [1, 2, 3, 4, 5, 6];
+		const result = shuffle(input);
+		expect([...result].sort()).toEqual(input);
+		expect(input).toEqual([1, 2, 3, 4, 5, 6]);
+	});
+});
diff --git a/web-player/src/lib/data/genres.test.ts b/web-player/src/lib/data/genres.test.ts
new file mode 100644
index 0000000..132f671
--- /dev/null
+++ b/web-player/src/lib/data/genres.test.ts
@@ -0,0 +1,28 @@
+import { describe, expect, it } from 'vitest';
+import { findConflict, genreHue } from './genres';
+
+describe('genreHue', () => {
+	it('is stable and within 0-359', () => {
+		for (const genre of ['rock', 'something-unknown', 'ñandú']) {
+			const hue = genreHue(genre);
+			expect(hue).toBe(genreHue(genre));
+			expect(hue).toBeGreaterThanOrEqual(0);
+			expect(hue).toBeLessThan(360);
+		}
+	});
+});
+
+describe('findConflict', () => {
+	const options = [
+		{ genre: 'a', incompatibleWith: ['b'] },
+		{ genre: 'b', incompatibleWith: ['a'] },
+		{ genre: 'c', incompatibleWith: [] }
+	];
+	it('returns the first incompatible pair', () => {
+		expect(findConflict(['c', 'a', 'b'], options)).toEqual(['a', 'b']);
+	});
+	it('returns undefined when compatible or unknown', () => {
+		expect(findConflict(['a', 'c'], options)).toBeUndefined();
+		expect(findConflict(['x', 'y'], options)).toBeUndefined();
+	});
+});
diff --git a/web-player/src/lib/data/search.test.ts b/web-player/src/lib/data/search.test.ts
new file mode 100644
index 0000000..d5f4373
--- /dev/null
+++ b/web-player/src/lib/data/search.test.ts
@@ -0,0 +1,28 @@
+import { describe, expect, it } from 'vitest';
+import { findTopResult } from './search';
+
+const empty = { tracks: [], albums: [], playlists: [], users: [] };
+
+describe('findTopResult', () => {
+	it('prefers prefix matches by kind: user, track, album, playlist', () => {
+		const result = findTopResult({
+			query: 'Ab',
+			users: [{ name: 'Zed' }, { name: 'abel' }],
+			tracks: [{ title: 'Abc' }],
+			albums: [{ title: 'Abd' }],
+			playlists: [{ name: 'Abe' }]
+		});
+		expect(result).toEqual({ kind: 'user', user: { name: 'abel' } });
+	});
+	it('matches case-insensitively and ignores surrounding spaces', () => {
+		const result = findTopResult({ ...empty, query: '  SO ', tracks: [{ title: 'song' }] });
+		expect(result).toEqual({ kind: 'track', track: { title: 'song' } });
+	});
+	it('falls back to the first available result', () => {
+		const result = findTopResult({ ...empty, query: 'zzz', albums: [{ title: 'Other' }] });
+		expect(result).toEqual({ kind: 'album', album: { title: 'Other' } });
+	});
+	it('returns null without results', () => {
+		expect(findTopResult({ ...empty, query: 'x' })).toBeNull();
+	});
+});
diff --git a/web-player/src/lib/player/player.svelte.ts b/web-player/src/lib/player/player.svelte.ts
index c56dc7b..6b34c37 100644
--- a/web-player/src/lib/player/player.svelte.ts
+++ b/web-player/src/lib/player/player.svelte.ts
@@ -4,6 +4,7 @@ import { extractAccent, type Accent } from '$lib/theme/palette';
 import { shuffle } from '$lib/data/collections';
 import { dialog } from '$lib/state/dialog.svelte';
 import { ListenTracker } from './listenTracker';
+import { append, insertNext, move, nextIndex, previousIndex } from './queue';
 
 export interface PlayerTrack {
 	id: string | number;
@@ -398,9 +399,7 @@ class PlayerState {
 			this.playQueue(items, 0);
 			return;
 		}
-		const tracks = [...this.tracks];
-		tracks.splice(this.#index() + 1, 0, ...items.map(toTrack));
-		this.tracks = tracks;
+		this.tracks = insertNext(this.tracks, this.#index(), items.map(toTrack));
 	}
 
 	appendToQueue(items: QueueItem[]) {
@@ -409,7 +408,7 @@ class PlayerState {
 			this.playQueue(items, 0);
 			return;
 		}
-		this.tracks = [...this.tracks, ...items.map(toTrack)];
+		this.tracks = append(this.tracks, items.map(toTrack));
 	}
 
 	playTrack(id: string | number) {
@@ -460,13 +459,7 @@ class PlayerState {
 	next() {
 		if (this.tracks.length === 0) return;
 		const idx = this.#index();
-		let nextIdx = (idx + 1) % this.tracks.length;
-		if (this.shuffle && this.tracks.length > 1) {
-			do {
-				nextIdx = Math.floor(Math.random() * this.tracks.length);
-			} while (nextIdx === idx);
-		}
-		this.currentId = this.tracks[nextIdx].id;
+		this.currentId = this.tracks[nextIndex(this.tracks.length, idx, this.shuffle)].id;
 		this.#loadCurrent();
 	}
 
@@ -490,13 +483,7 @@ class PlayerState {
 	}
 
 	moveQueueItem(from: number, insertAt: number) {
-		if (from < 0 || from >= this.tracks.length) return;
-		const target = insertAt > from ? insertAt - 1 : insertAt;
-		if (target === from) return;
-		const tracks = [...this.tracks];
-		const [item] = tracks.splice(from, 1);
-		tracks.splice(target, 0, item);
-		this.tracks = tracks;
+		this.tracks = move(this.tracks, from, insertAt);
 	}
 
 	clearUpcoming() {
@@ -511,7 +498,7 @@ class PlayerState {
 			return;
 		}
 		const idx = this.#index();
-		this.currentId = this.tracks[(idx - 1 + this.tracks.length) % this.tracks.length].id;
+		this.currentId = this.tracks[previousIndex(this.tracks.length, idx)].id;
 		this.#loadCurrent();
 	}
 
diff --git a/web-player/src/lib/player/queue.test.ts b/web-player/src/lib/player/queue.test.ts
new file mode 100644
index 0000000..b6ac598
--- /dev/null
+++ b/web-player/src/lib/player/queue.test.ts
@@ -0,0 +1,60 @@
+import { describe, expect, it } from 'vitest';
+import { append, insertNext, move, nextIndex, previousIndex } from './queue';
+
+describe('insertNext', () => {
+	it('inserts right after the given index without mutating', () => {
+		const queue = ['a', 'b', 'c'];
+		expect(insertNext(queue, 0, ['x', 'y'])).toEqual(['a', 'x', 'y', 'b', 'c']);
+		expect(queue).toEqual(['a', 'b', 'c']);
+	});
+	it('inserts at the start when index is -1', () => {
+		expect(insertNext(['a'], -1, ['x'])).toEqual(['x', 'a']);
+	});
+});
+
+describe('append', () => {
+	it('adds items at the end', () => {
+		expect(append(['a'], ['b', 'c'])).toEqual(['a', 'b', 'c']);
+	});
+});
+
+describe('move', () => {
+	const queue = ['a', 'b', 'c', 'd'];
+	it('moves an item forward using insert position semantics', () => {
+		expect(move(queue, 0, 3)).toEqual(['b', 'c', 'a', 'd']);
+	});
+	it('moves an item backward', () => {
+		expect(move(queue, 3, 1)).toEqual(['a', 'd', 'b', 'c']);
+	});
+	it('is a no-op when the target equals the origin', () => {
+		expect(move(queue, 1, 1)).toBe(queue);
+		expect(move(queue, 1, 2)).toBe(queue);
+	});
+	it('ignores out-of-range origins', () => {
+		expect(move(queue, -1, 0)).toBe(queue);
+		expect(move(queue, 4, 0)).toBe(queue);
+	});
+});
+
+describe('nextIndex', () => {
+	it('advances and wraps around', () => {
+		expect(nextIndex(3, 0, false)).toBe(1);
+		expect(nextIndex(3, 2, false)).toBe(0);
+	});
+	it('never repeats the current index when shuffling', () => {
+		const values = [0.4, 0.5, 0.9];
+		let call = 0;
+		expect(nextIndex(3, 1, true, () => values[call++])).toBe(2);
+		expect(call).toBe(3);
+	});
+	it('ignores shuffle with a single track', () => {
+		expect(nextIndex(1, 0, true)).toBe(0);
+	});
+});
+
+describe('previousIndex', () => {
+	it('steps back and wraps around', () => {
+		expect(previousIndex(3, 2)).toBe(1);
+		expect(previousIndex(3, 0)).toBe(2);
+	});
+});
diff --git a/web-player/src/lib/player/queue.ts b/web-player/src/lib/player/queue.ts
new file mode 100644
index 0000000..cee8b9a
--- /dev/null
+++ b/web-player/src/lib/player/queue.ts
@@ -0,0 +1,39 @@
+export function insertNext<T>(queue: T[], afterIndex: number, items: T[]): T[] {
+	const result = [...queue];
+	result.splice(afterIndex + 1, 0, ...items);
+	return result;
+}
+
+export function append<T>(queue: T[], items: T[]): T[] {
+	return [...queue, ...items];
+}
+
+export function move<T>(queue: T[], from: number, insertAt: number): T[] {
+	if (from < 0 || from >= queue.length) return queue;
+	const target = insertAt > from ? insertAt - 1 : insertAt;
+	if (target === from) return queue;
+	const result = [...queue];
+	const [item] = result.splice(from, 1);
+	result.splice(target, 0, item);
+	return result;
+}
+
+export function nextIndex(
+	length: number,
+	index: number,
+	shuffle: boolean,
+	random: () => number = Math.random
+): number {
+	if (shuffle && length > 1) {
+		let next: number;
+		do {
+			next = Math.floor(random() * length);
+		} while (next === index);
+		return next;
+	}
+	return (index + 1) % length;
+}
+
+export function previousIndex(length: number, index: number): number {
+	return (index - 1 + length) % length;
+}
diff --git a/web-player/src/lib/server/albumForm.test.ts b/web-player/src/lib/server/albumForm.test.ts
new file mode 100644
index 0000000..a6dd02c
--- /dev/null
+++ b/web-player/src/lib/server/albumForm.test.ts
@@ -0,0 +1,36 @@
+import { describe, expect, it } from 'vitest';
+import { ALBUM_EARLIEST_YEAR } from '$lib/config';
+import { parseAlbumForm } from './albumForm';
+
+function form(fields: Record<string, string>): FormData {
+	const data = new FormData();
+	for (const [key, value] of Object.entries(fields)) data.set(key, value);
+	return data;
+}
+
+describe('parseAlbumForm', () => {
+	it('parses a valid form and trims values', () => {
+		expect(
+			parseAlbumForm(form({ title: ' Title ', description: ' d ', releaseYear: '2020' }))
+		).toEqual({ body: { title: 'Title', description: 'd', releaseYear: 2020 } });
+	});
+	it('turns empty optional fields into null', () => {
+		expect(parseAlbumForm(form({ title: 'T' }))).toEqual({
+			body: { title: 'T', description: null, releaseYear: null }
+		});
+	});
+	it('rejects empty or too long titles', () => {
+		expect(parseAlbumForm(form({ title: '  ' }))).toHaveProperty('failMessage');
+		expect(parseAlbumForm(form({ title: 'x'.repeat(201) }))).toHaveProperty('failMessage');
+	});
+	it('rejects too long descriptions', () => {
+		expect(parseAlbumForm(form({ title: 'T', description: 'x'.repeat(257) }))).toHaveProperty(
+			'failMessage'
+		);
+	});
+	it('rejects invalid years', () => {
+		for (const releaseYear of ['abc', '2020.5', String(ALBUM_EARLIEST_YEAR - 1), '9999']) {
+			expect(parseAlbumForm(form({ title: 'T', releaseYear }))).toHaveProperty('failMessage');
+		}
+	});
+});
diff --git a/web-player/src/lib/server/greeting.test.ts b/web-player/src/lib/server/greeting.test.ts
new file mode 100644
index 0000000..8779eb0
--- /dev/null
+++ b/web-player/src/lib/server/greeting.test.ts
@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest';
+import { pickGreeting } from './greeting';
+
+const BIRTHDAY = 'Hoy toca celebrar. Tú eliges la música.';
+
+describe('pickGreeting', () => {
+	it('uses the birthday message over special dates', () => {
+		const date = new Date(2026, 11, 25, 10);
+		expect(pickGreeting(date, { month: 12, day: 25 })).toBe(BIRTHDAY);
+	});
+	it('uses the special date message', () => {
+		expect(pickGreeting(new Date(2026, 11, 25, 10))).toBe('Feliz Navidad. Pon tu banda sonora.');
+	});
+	it('returns a non-empty schedule message at every hour', () => {
+		for (let hour = 0; hour < 24; hour++) {
+			expect(pickGreeting(new Date(2026, 2, 10, hour)).length).toBeGreaterThan(0);
+		}
+	});
+	it('ignores a birthday on another day', () => {
+		expect(pickGreeting(new Date(2026, 2, 10, 10), { month: 1, day: 2 })).not.toBe(BIRTHDAY);
+	});
+});
diff --git a/web-player/src/lib/utils/format.test.ts b/web-player/src/lib/utils/format.test.ts
new file mode 100644
index 0000000..106ae7a
--- /dev/null
+++ b/web-player/src/lib/utils/format.test.ts
@@ -0,0 +1,46 @@
+import { describe, expect, it } from 'vitest';
+import { albumMeta, fmtDurationLong, fmtTime, plural } from './format';
+
+describe('fmtTime', () => {
+	it('formats minutes and padded seconds', () => {
+		expect(fmtTime(0)).toBe('0:00');
+		expect(fmtTime(65)).toBe('1:05');
+		expect(fmtTime(600)).toBe('10:00');
+	});
+	it('floors fractions and clamps invalid input', () => {
+		expect(fmtTime(59.9)).toBe('0:59');
+		expect(fmtTime(-5)).toBe('0:00');
+		expect(fmtTime(NaN)).toBe('0:00');
+	});
+});
+
+describe('fmtDurationLong', () => {
+	it('shows minutes only under an hour', () => {
+		expect(fmtDurationLong(59)).toBe('0 Min');
+		expect(fmtDurationLong(125)).toBe('2 Min');
+	});
+	it('shows hours and minutes', () => {
+		expect(fmtDurationLong(3600)).toBe('1 H 0 Min');
+		expect(fmtDurationLong(3600 * 2 + 60 * 5)).toBe('2 H 5 Min');
+	});
+});
+
+describe('plural', () => {
+	it('uses the singular only for 1', () => {
+		expect(plural(1, 'canción', 'canciones')).toBe('1 canción');
+		expect(plural(0, 'canción', 'canciones')).toBe('0 canciones');
+		expect(plural(2, 'canción', 'canciones')).toBe('2 canciones');
+	});
+});
+
+describe('albumMeta', () => {
+	it('joins year and track count', () => {
+		expect(albumMeta(2020, 1)).toBe('2020 · 1 canción');
+		expect(albumMeta(2020, 12)).toBe('2020 · 12 canciones');
+	});
+	it('omits missing parts', () => {
+		expect(albumMeta(undefined, 3)).toBe('3 canciones');
+		expect(albumMeta(2020)).toBe('2020');
+		expect(albumMeta()).toBe('');
+	});
+});
diff --git a/web-player/vite.config.ts b/web-player/vite.config.ts
index ed6aabd..6327189 100644
--- a/web-player/vite.config.ts
+++ b/web-player/vite.config.ts
@@ -1,9 +1,12 @@
 import tailwindcss from '@tailwindcss/vite';
 import adapter from '@sveltejs/adapter-node';
 import { sveltekit } from '@sveltejs/kit/vite';
-import { defineConfig } from 'vite';
+import { defineConfig } from 'vitest/config';
 
 export default defineConfig({
+	test: {
+		include: ['src/**/*.test.ts']
+	},
 	server: {
 		port: process.env.PORT ? Number(process.env.PORT) : 5173
 	},