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/package.json

@@ -11,11 +11,16 @@
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
+    "@radix-ui/react-slot": "^1.0.2",
+    "@tanstack/react-query": "^5.40.0",
+    "axios": "^1.7.2",
+    "class-variance-authority": "^0.7.0",
+    "clsx": "^2.1.0",
+    "lucide-react": "^0.400.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-router-dom": "^6.23.1",
-    "@tanstack/react-query": "^5.40.0",
-    "axios": "^1.7.2"
+    "tailwind-merge": "^2.3.0"
   },
   "devDependencies": {
     "@types/react": "^18.3.3",
@@ -23,9 +28,12 @@
     "@typescript-eslint/eslint-plugin": "^7.13.0",
     "@typescript-eslint/parser": "^7.13.0",
     "@vitejs/plugin-react": "^4.3.1",
+    "autoprefixer": "^10.4.0",
     "eslint": "^8.57.0",
     "eslint-plugin-react-hooks": "^4.6.2",
     "eslint-plugin-react-refresh": "^0.4.7",
+    "postcss": "^8.4.0",
+    "tailwindcss": "^3.4.0",
     "typescript": "^5.4.5",
     "vite": "^5.3.1"
   }