refactor: rename frontend/ to web-player/ and format with prettier

Updates the compose build context, the Dockerfile header, the package
name and the docs. The compose service and container keep the name
frontend/musify-frontend, since nginx routes to http://frontend:3000.

The source files also carry the first prettier pass, so the rename and
the reformat land together for them.

web-player/src/lib/components/ui/Cover.svelte

@@ -0,0 +1,52 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte';
+	import { gradientForHue } from '$lib/theme/color';
+
+	interface Props {
+		trackId: string | number;
+		hue: number;
+		size?: 'small' | 'medium' | 'large';
+		src?: string;
+		alt?: string;
+		class?: string;
+		children?: Snippet;
+	}
+
+	let {
+		trackId,
+		hue,
+		size = 'medium',
+		src,
+		alt = '',
+		class: klass = '',
+		children
+	}: Props = $props();
+
+	let failed = $state(false);
+	let loaded = $state(false);
+
+	const imageSrc = $derived(src ?? `/api/tracks/${trackId}/cover?size=${size}`);
+
+	$effect(() => {
+		imageSrc;
+		failed = false;
+		loaded = false;
+	});
+</script>
+
+<div
+	class="relative overflow-hidden {klass}"
+	style={loaded && !failed ? undefined : `background:${gradientForHue(hue)}`}
+>
+	{#if !failed}
+		<img
+			src={imageSrc}
+			{alt}
+			loading="lazy"
+			onload={() => (loaded = true)}
+			onerror={() => (failed = true)}
+			class="h-full w-full object-cover"
+		/>
+	{/if}
+	{@render children?.()}
+</div>