Add: SignalR real-time notifications for admin panel

- Created NotificationHub for instant browser notifications
- Updated CryptoPaymentService to broadcast via SignalR
- Added JavaScript client with toast notifications
- Works with custom SSL certificates (no FCM dependency)
- Automatic reconnection with exponential backoff
- Notification sound and visual indicators
- Bypasses all Web Push SSL certificate issues

LittleShop/Areas/Admin/Views/Shared/_Layout.cshtml

@@ -159,6 +159,9 @@
     <script src="/js/blazor-integration.js"></script>
     <script src="/js/pwa.js"></script>
     <script src="/js/notifications.js"></script>
+    <!-- SignalR Real-Time Notifications -->
+    <script src="https://cdn.jsdelivr.net/npm/@@microsoft/signalr@@latest/dist/browser/signalr.min.js"></script>
+    <script src="/js/signalr-notifications.js"></script>
     <script src="/js/modern-mobile.js"></script>
     @await RenderSectionAsync("Scripts", required: false)
     <!-- Mobile Bottom Navigation -->