perf(auth): eliminate login page flicker by moving session check to App.tsx

src/App.tsx

@@ -8,7 +8,7 @@ import GeneratePage from './pages/Generate';
 import ManagePage from './pages/Manage';
 import ProfilePage from './pages/Profile';
 
-const PrivateRoute = () => {
+const PrivatePrivateRoute = () => {
   const user = authApi.getCurrentUser();
   if (!user) {
     return <Navigate to="/login" replace />;
@@ -16,6 +16,14 @@ const PrivateRoute = () => {
   return <Outlet />;
 };
 
+const PublicRoute = ({ children }: { children: React.ReactNode }) => {
+  const user = authApi.getCurrentUser();
+  if (user) {
+    return <Navigate to="/admin/dashboard" replace />;
+  }
+  return <>{children}</>;
+};
+
 const AdminRoutes = () => {
   return (
     <AdminLayout>
@@ -29,7 +37,11 @@ function App() {
     <BrowserRouter>
       <Routes>
         <Route path="/" element={<Navigate to="/login" replace />} />
-        <Route path="/login" element={<LoginPage />} />
+        <Route path="/login" element={
+          <PublicRoute>
+            <LoginPage />
+          </PublicRoute>
+        } />
         <Route path="/query" element={<PublicQueryPage />} />
         
         <Route element={<PrivateRoute />}>
@@ -46,4 +58,4 @@ function App() {
   );
 }
 
-export default App;
+export default App;