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/ExplicitBadge.svelte

@@ -0,0 +1,13 @@
+<script lang="ts">
+	interface Props {
+		class?: string;
+	}
+
+	let { class: klass = '' }: Props = $props();
+</script>
+
+<span
+	class="grid h-4 w-4 shrink-0 place-items-center rounded-sm bg-fg-3 text-[11px] leading-4 font-bold text-bg {klass}"
+	title="Contenido explícito"
+	aria-label="Contenido explícito">E</span
+>