feat: Add OIDC authentication with Authentik integration

- Add OIDC login flow with Authentik provider
- Implement session-based auth with Redis store
- Add avatar display from OIDC claims
- Fix input field performance with react-textarea-autosize
- Stabilize callbacks to prevent unnecessary re-renders
- Fix history loading to skip empty session files
- Add 2-row default height for input textarea

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/App.jsx

@@ -1,10 +1,12 @@
 import { useState, useCallback, useEffect } from 'react';
+import { AuthProvider, useAuth } from './contexts/AuthContext';
 import { SessionProvider, useSessionManager } from './contexts/SessionContext';
 import { Sidebar } from './components/Sidebar';
 import { TabBar } from './components/TabBar';
 import { ChatPanel } from './components/ChatPanel';
 import { SplitLayout } from './components/SplitLayout';
-import { Menu } from 'lucide-react';
+import { LoginPage } from './components/LoginPage';
+import { Menu, Loader2 } from 'lucide-react';
 
 function AppContent() {
   const {
@@ -101,7 +103,30 @@ function AppContent() {
   );
 }
 
-function App() {
+// Loading screen while checking auth
+function LoadingScreen() {
+  return (
+    <div className="min-h-screen bg-dark-950 flex items-center justify-center">
+      <div className="text-center">
+        <Loader2 className="w-8 h-8 text-orange-500 animate-spin mx-auto mb-4" />
+        <p className="text-dark-400">Loading...</p>
+      </div>
+    </div>
+  );
+}
+
+// Auth wrapper - shows login or main app
+function AuthenticatedApp() {
+  const { isAuthenticated, loading } = useAuth();
+
+  if (loading) {
+    return <LoadingScreen />;
+  }
+
+  if (!isAuthenticated) {
+    return <LoginPage />;
+  }
+
   return (
     <SessionProvider>
       <AppContent />
@@ -109,4 +134,12 @@ function App() {
   );
 }
 
+function App() {
+  return (
+    <AuthProvider>
+      <AuthenticatedApp />
+    </AuthProvider>
+  );
+}
+
 export default App;