Restyle to light content theme with Lucide icons

- index.css: light body (#f4f5f7), white cards, sidebar stays dark navy
- AppIcon.tsx: Lucide icon lookup component for app tiles and nav
- Sidebar.tsx: Lucide nav icons (LayoutGrid, Users, Activity, LogOut)
- Dashboard.tsx: white cards with shadows, coloured Lucide app icons
- AdminMonitor.tsx: light theme throughout, Lucide tab icons,
  better status colours for light backgrounds

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

package.json

@@ -9,6 +9,7 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "lucide-react": "^1.23.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-router-dom": "^6.28.1"