integrate backend Google OAuth and set up avatar media proxy

- Implement the full login and onboarding workflow to handle name, handle, and avatar.

- Rewrite avatar asset URLs to route through the self-hosted media service.

- Add Next.js rewrites configuration to proxy media requests directly to the FastAPI backend.

app/(dashboard)/page.tsx

@@ -0,0 +1,8 @@
+// app/page.tsx
+// 这个页面只有在 已登录 且 路径为 / 时才会被 proxy 放行到这里
+import DiscoverPage from "./discover/page"; 
+
+export default function RootPage() {
+  // 直接显示 Discover 的内容即可，不要再判断 Cookie 和 redirect 了
+  return <DiscoverPage />;
+}