feat: auth, persistence, and AI integration

- Cookie-based auth via backend proxy (httpOnly JWTs)
- Supabase Postgres persistence for sessions/messages/profiles + RLS
- Fix cross-user session leak (token cache keyed by full token, not 50-char prefix)
- Fix missing table grants (42501) via migration; auto-provision profiles on user creation
- Chat validation, ownership checks, rate limiting, /api/chat/sessions route ordering
- Frontend auth-state reset + credentials include
- OpenRouter AI provider (OpenAI-compatible base URL, reasoning disabled)
- Tests: chatValidation, appState

frontend/src/main.jsx

@@ -5,10 +5,13 @@
 import React from "react";
 import ReactDOM from "react-dom/client";
 import App from "./App.jsx";
+import { AuthProvider } from "./lib/auth/SupabaseAuth.jsx";
 import "./styles/global.css";
 
 ReactDOM.createRoot(document.getElementById("root")).render(
   <React.StrictMode>
-    <App />
+    <AuthProvider>
+      <App />
+    </AuthProvider>
   </React.StrictMode>
 );