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/login/+page.server.ts

@@ -0,0 +1,9 @@
+import { redirect } from '@sveltejs/kit';
+import type { PageServerLoad } from './$types';
+
+export const load: PageServerLoad = ({ locals, url }) => {
+	if (locals.user) {
+		redirect(302, url.searchParams.get('returnTo') ?? '/');
+	}
+	return { returnTo: url.searchParams.get('returnTo') ?? '/' };
+};