fix: add auth guard redirect and wire logout into NavAccountCard

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/layout/auth-guard.tsx

@@ -0,0 +1,12 @@
+import { Navigate, Outlet } from 'react-router';
+import { useAuth } from '@/providers/auth-provider';
+
+export const AuthGuard = () => {
+    const { isAuthenticated } = useAuth();
+
+    if (!isAuthenticated) {
+        return <Navigate to="/login" replace />;
+    }
+
+    return <Outlet />;
+};