Implement shadcn/ui + Tailwind CSS UI layer

- Design token system via CSS custom properties (light/dark mode)
- Theme context hook + ThemeToggle component
- AppShell + collapsible Sidebar replace inline Nav
- LoginPage redesigned: two-column grid with hero panel
- shadcn/ui Button and Input components
- Tailwind config wired to CSS variable tokens
- All pages de-Nav'd; PrivateRoute/AdminRoute wrap with AppShell
- TypeScript passes clean (npm run typecheck)

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

frontend/src/App.tsx

@@ -2,6 +2,7 @@ import { Routes, Route, Navigate } from "react-router-dom";
 import { useQuery } from "@tanstack/react-query";
 import { useAuth } from "./hooks/useAuth";
 import { getMe } from "./api/client";
+import AppShell from "./components/AppShell";
 import LoginPage from "./pages/LoginPage";
 import DashboardPage from "./pages/DashboardPage";
 import ProfilePage from "./pages/ProfilePage";
@@ -13,7 +14,11 @@ import AIAdminSettingsPage from "./pages/AIAdminSettingsPage";
 
 function PrivateRoute({ children }: { children: React.ReactNode }) {
   const { token } = useAuth();
-  return token ? <>{children}</> : <Navigate to="/login" replace />;
+  return token ? (
+    <AppShell>{children}</AppShell>
+  ) : (
+    <Navigate to="/login" replace />
+  );
 }
 
 function AdminRoute({ children }: { children: React.ReactNode }) {
@@ -25,7 +30,7 @@ function AdminRoute({ children }: { children: React.ReactNode }) {
   if (isLoading) return null;
   // Redirect to /login (not /) so the route appears not to exist
   if (!user?.is_admin) return <Navigate to="/login" replace />;
-  return <>{children}</>;
+  return <AppShell>{children}</AppShell>;
 }
 
 export default function App() {