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/PageHeader.svelte

@@ -0,0 +1,24 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte';
+
+	interface Props {
+		title: string;
+		subtitle?: string;
+		class?: string;
+		actions?: Snippet;
+	}
+
+	let { title, subtitle, class: klass = '', actions }: Props = $props();
+</script>
+
+<div class="flex items-end justify-between gap-4 {klass}">
+	<div class="min-w-0">
+		<h1 class="text-3xl font-bold tracking-tight text-fg sm:text-4xl">{title}</h1>
+		{#if subtitle}
+			<p class="mt-2 text-fg-2">{subtitle}</p>
+		{/if}
+	</div>
+	{#if actions}
+		<div class="flex shrink-0 items-center gap-3">{@render actions()}</div>
+	{/if}
+</div>