feat: add Supabase email auth with signup/login gate

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.tsx

@@ -2,6 +2,7 @@ import React from 'react';
 import ReactDOM from 'react-dom/client';
 import App from './App';
 import ErrorBoundary from './components/ErrorBoundary';
+import { AuthGate } from './components/AuthGate';
 
 const rootElement = document.getElementById('root');
 if (!rootElement) {
@@ -11,6 +12,8 @@ if (!rootElement) {
 const root = ReactDOM.createRoot(rootElement);
 root.render(
   <ErrorBoundary>
-    <App />
+    <AuthGate>
+      <App />
+    </AuthGate>
   </ErrorBoundary>
 );