Enhance map update handling and connection stability

- Introduced mechanisms to detect stale connections in the map updates, allowing for automatic reconnection if no messages are received within a specified timeframe.
- Updated the `refresh` method in `SmartTileLayer` to return a boolean indicating whether the tile was refreshed, improving the handling of tile updates.
- Enhanced the `Send` method in the `Topic` struct to drop messages for full subscribers while keeping them subscribed, ensuring continuous delivery of future updates.
- Added a keepalive mechanism in the `WatchGridUpdates` handler to maintain the connection and prevent timeouts.

internal/app/topic.go

@@ -15,7 +15,9 @@ func (t *Topic[T]) Watch(c chan *T) {
 	t.c = append(t.c, c)
 }
 
-// Send broadcasts to all subscribers.
+// Send broadcasts to all subscribers. If a subscriber's channel is full,
+// the message is dropped for that subscriber only; the subscriber is not
+// removed, so the connection stays alive and later updates are still delivered.
 func (t *Topic[T]) Send(b *T) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
@@ -23,9 +25,7 @@ func (t *Topic[T]) Send(b *T) {
 		select {
 		case t.c[i] <- b:
 		default:
-			close(t.c[i])
-			t.c[i] = t.c[len(t.c)-1]
-			t.c = t.c[:len(t.c)-1]
+			// Channel full: drop this message for this subscriber, keep them subscribed
 		}
 	}
 }