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]/stream/+server.ts

@@ -0,0 +1,14 @@
+import type { RequestHandler } from './$types';
+import { json } from '@sveltejs/kit';
+import { createApiClient, requireAccessToken, unwrapOrError } from '$lib/server/api';
+
+export const GET: RequestHandler = async ({ params, locals, fetch }) => {
+	const accessToken = requireAccessToken(locals, 'Inicia sesión para reproducir.');
+
+	const api = createApiClient({ fetch, accessToken });
+	const result = await api.GET('/tracks/{id}/stream', {
+		params: { path: { id: params.id } }
+	});
+
+	return json(unwrapOrError(result, 'No se pudo obtener el stream de la pista.'));
+};