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/routes/api/tracks/[id]/cover/+server.ts

@@ -0,0 +1,6 @@
+import type { RequestHandler } from './$types';
+import { proxyCoverImage } from '$lib/server/image';
+
+export const GET: RequestHandler = async ({ params, url, fetch }) => {
+	return proxyCoverImage(fetch, 'tracks', params.id, url.searchParams.get('size'));
+};