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/app/layout.tsx

@@ -2,6 +2,7 @@ import type { Metadata } from "next";
 import { Geist } from "next/font/google";
 import "./globals.css";
 import { Sidebar } from "@/components/sidebar";
+import { Toaster } from "sonner";
 
 const geistSans = Geist({
   variable: "--font-geist-sans",
@@ -27,6 +28,17 @@ export default function RootLayout({
             {children}
           </main>
         </div>
+        <Toaster 
+          theme="dark" 
+          position="top-right"
+          toastOptions={{
+            style: {
+              background: '#171717',
+              border: '1px solid #262626',
+              color: '#fff',
+            },
+          }}
+        />
       </body>
     </html>
   );