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/PlayPauseOverlay.svelte

@@ -0,0 +1,23 @@
+<script lang="ts">
+	import Play from '@lucide/svelte/icons/play';
+	import Pause from '@lucide/svelte/icons/pause';
+
+	interface Props {
+		active: boolean;
+		playing: boolean;
+	}
+
+	let { active, playing }: Props = $props();
+</script>
+
+<span
+	class="absolute right-2.5 bottom-2.5 grid h-11 w-11 translate-y-2 place-items-center rounded-full bg-accent-soft text-on-accent opacity-0 shadow-lg transition-all group-hover:translate-y-0 group-hover:opacity-100"
+	class:!opacity-100={active}
+	class:!translate-y-0={active}
+>
+	{#if active && playing}
+		<Pause class="h-5 w-5" fill="currentColor" />
+	{:else}
+		<Play class="h-5 w-5" fill="currentColor" />
+	{/if}
+</span>