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/pages/AdminPage.tsx

@@ -9,7 +9,6 @@ import {
   type AdminUserCreate,
   type UserData,
 } from "../api/client";
-import Nav from "../components/Nav";
 
 export default function AdminPage() {
   const queryClient = useQueryClient();
@@ -68,7 +67,6 @@ export default function AdminPage() {
 
   return (
     <>
-      <Nav />
       <div style={{ padding: 32, maxWidth: 800 }}>
         <h1>User Management</h1>