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/TrackIndexCell.svelte

@@ -0,0 +1,27 @@
+<script lang="ts">
+	import Play from '@lucide/svelte/icons/play';
+	import NowPlaying from './NowPlaying.svelte';
+
+	interface Props {
+		index: number;
+		active: boolean;
+		playing: boolean;
+		onToggle: () => void;
+	}
+
+	let { index, active, playing, onToggle }: Props = $props();
+</script>
+
+<button
+	type="button"
+	onclick={onToggle}
+	aria-label={active && playing ? 'Pausar' : 'Reproducir'}
+	class="relative grid h-8 w-8 place-items-center overflow-hidden rounded text-sm text-muted"
+>
+	{#if active}
+		<NowPlaying paused={!playing} />
+	{:else}
+		<span class="group-hover:hidden">{index + 1}</span>
+		<Play class="hidden h-3.5 w-3.5 text-fg group-hover:block" fill="currentColor" />
+	{/if}
+</button>