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/SectionHeading.svelte

@@ -0,0 +1,10 @@
+<script lang="ts">
+	interface Props {
+		title: string;
+		class?: string;
+	}
+
+	let { title, class: klass = '' }: Props = $props();
+</script>
+
+<h2 class="mb-6 text-2xl font-bold text-fg {klass}">{title}</h2>