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.

providers/index.tsx

@@ -0,0 +1,13 @@
+"use client";
+
+import React from "react";
+import { GraphqlProvider } from "./ApolloProvider";
+
+export function Providers({ children }: { children: React.ReactNode }) {
+  return (
+    <GraphqlProvider>
+      {/* 在这里可以继续嵌套其他全局 Provider */}
+      {children}
+    </GraphqlProvider>
+  );
+}