Document the library components and test dropped folder reading

web-player/src/lib/data/filesFromDrop.test.ts

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+import { describe, expect, it } from 'vitest';
+import { filesFromDrop } from './uploadQueue';
+
+function fileEntry(name: string) {
+	const file = new File(['x'], name);
+	return {
+		isFile: true,
+		isDirectory: false,
+		file: (resolve: (file: File) => void) => resolve(file)
+	};
+}
+
+function directoryEntry(children: unknown[]) {
+	let served = false;
+	return {
+		isFile: false,
+		isDirectory: true,
+		createReader: () => ({
+			readEntries: (resolve: (entries: unknown[]) => void) => {
+				resolve(served ? [] : children);
+				served = true;
+			}
+		})
+	};
+}
+
+function transfer(entries: unknown[], files: File[] = []) {
+	return {
+		items: entries.map((entry) => ({ webkitGetAsEntry: () => entry })),
+		files
+	} as unknown as DataTransfer;
+}
+
+describe('filesFromDrop', () => {
+	it('collects files from nested folders', async () => {
+		const dropped = transfer([
+			fileEntry('a.mp3'),
+			directoryEntry([fileEntry('b.mp3'), directoryEntry([fileEntry('c.mp3')])])
+		]);
+		const files = await filesFromDrop(dropped);
+		expect(files.map((file) => file.name).sort()).toEqual(['a.mp3', 'b.mp3', 'c.mp3']);
+	});
+
+	it('falls back to the plain file list without entries', async () => {
+		const plain = new File(['x'], 'd.mp3');
+		const dropped = {
+			items: [{ webkitGetAsEntry: () => null }],
+			files: [plain]
+		} as unknown as DataTransfer;
+		expect(await filesFromDrop(dropped)).toEqual([plain]);
+	});
+});