feat: implement beautiful toast notifications with Sonner

- Install sonner package for toast notifications
- Add Toaster component to root layout with dark theme styling
- Replace all alert() calls with toast.success() and toast.error()
- Add descriptive messages for success states
- Toast notifications match the app's dark theme design

admin-web/package.json

@@ -20,6 +20,7 @@
     "radix-ui": "^1.4.3",
     "react": "19.2.3",
     "react-dom": "19.2.3",
+    "sonner": "^2.0.7",
     "tailwind-merge": "^3.5.0"
   },
   "devDependencies": {