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/EmptyState.svelte

@@ -0,0 +1,31 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte';
+	import type { LucideIcon } from '@lucide/svelte';
+
+	interface Props {
+		icon?: LucideIcon;
+		title?: string;
+		description?: string;
+		class?: string;
+		actions?: Snippet;
+	}
+
+	let { icon: Icon, title, description, class: klass = '', actions }: Props = $props();
+</script>
+
+<div class="flex flex-col items-center px-6 py-14 text-center {klass}">
+	{#if Icon}
+		<span class="grid h-14 w-14 place-items-center rounded-full bg-accent-soft text-on-accent">
+			<Icon class="h-6 w-6" />
+		</span>
+	{/if}
+	{#if title}
+		<h3 class="mt-5 text-2xl font-bold tracking-tight text-fg">{title}</h3>
+	{/if}
+	{#if description}
+		<p class="mt-2 text-fg-2">{description}</p>
+	{/if}
+	{#if actions}
+		<div class="mt-6 flex items-center gap-3">{@render actions()}</div>
+	{/if}
+</div>