Add admin dashboard shell with sidebar navigation and page scaffolds.

Replace the default Next.js landing page with a client-side AppShell layout, placeholder pages for each admin section, and UI utility dependencies.

Co-authored-by: Cursor <cursoragent@cursor.com>

package.json

@@ -9,9 +9,12 @@
     "lint": "eslint"
   },
   "dependencies": {
+    "clsx": "^2.1.1",
+    "lucide-react": "^1.17.0",
     "next": "16.2.7",
     "react": "19.2.4",
-    "react-dom": "19.2.4"
+    "react-dom": "19.2.4",
+    "tailwind-merge": "^3.6.0"
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4",