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/AppsPage.tsx

@@ -1,6 +1,5 @@
 import { Link } from "react-router-dom";
 import { useQuery } from "@tanstack/react-query";
-import Nav from "../components/Nav";
 import { getMe } from "../api/client";
 
 interface AppCard {
@@ -36,7 +35,6 @@ export default function AppsPage() {
 
   return (
     <>
-      <Nav />
       <div style={{ padding: 32, maxWidth: 900, margin: "0 auto" }}>
         <h1>Apps</h1>
         <div style={{ display: "flex", gap: 24, flexWrap: "wrap", marginTop: 24 }}>
@@ -102,3 +100,4 @@ export default function AppsPage() {
     </>
   );
 }
+