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/Alert.svelte

@@ -0,0 +1,20 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte';
+
+	interface Props {
+		tone?: 'neutral' | 'danger';
+		class?: string;
+		children: Snippet;
+	}
+
+	let { tone = 'neutral', class: klass = '', children }: Props = $props();
+
+	const tones = {
+		neutral: 'border-line bg-surface text-fg-2',
+		danger: 'border-danger/40 bg-danger/10 text-danger-fg'
+	};
+</script>
+
+<p class="rounded-control border px-4 py-3 text-base {tones[tone]} {klass}">
+	{@render children()}
+</p>