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/Hubs/NotificationHub.cs

@@ -0,0 +1,39 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.SignalR;
+
+namespace LittleShop.Hubs;
+
+[Authorize(Roles = "Admin")]
+public class NotificationHub : Hub
+{
+    private readonly ILogger<NotificationHub> _logger;
+
+    public NotificationHub(ILogger<NotificationHub> logger)
+    {
+        _logger = logger;
+    }
+
+    public override async Task OnConnectedAsync()
+    {
+        _logger.LogInformation("Admin user connected to notification hub: {ConnectionId}", Context.ConnectionId);
+        await base.OnConnectedAsync();
+    }
+
+    public override async Task OnDisconnectedAsync(Exception? exception)
+    {
+        _logger.LogInformation("Admin user disconnected from notification hub: {ConnectionId}", Context.ConnectionId);
+        await base.OnDisconnectedAsync(exception);
+    }
+
+    // Client can call this to test notifications
+    public async Task SendTestNotification()
+    {
+        await Clients.Caller.SendAsync("ReceiveNotification", new
+        {
+            title = "Test Notification",
+            message = "This is a test notification from SignalR!",
+            type = "info",
+            timestamp = DateTime.UtcNow
+        });
+    }
+}