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/+layout.server.ts

@@ -0,0 +1,11 @@
+import { redirect } from '@sveltejs/kit';
+import type { LayoutServerLoad } from './$types';
+
+const PUBLIC_PATHS = new Set(['/login']);
+
+export const load: LayoutServerLoad = ({ locals, url }) => {
+	if (!locals.user && !PUBLIC_PATHS.has(url.pathname)) {
+		redirect(302, `/login?returnTo=${encodeURIComponent(url.pathname + url.search)}`);
+	}
+	return { user: locals.user };
+};