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/Surface.svelte

@@ -0,0 +1,17 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte';
+
+	interface Props {
+		padding?: 'sm' | 'md' | 'lg';
+		class?: string;
+		children: Snippet;
+	}
+
+	let { padding = 'md', class: klass = '', children }: Props = $props();
+
+	const paddings = { sm: 'p-4', md: 'p-6', lg: 'p-12' };
+</script>
+
+<div class="rounded-panel border border-line bg-surface transition {paddings[padding]} {klass}">
+	{@render children()}
+</div>