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/app_test.go

@@ -93,16 +93,41 @@ func TestTopicClose(t *testing.T) {
 	}
 }
 
-func TestTopicDropsSlowSubscriber(t *testing.T) {
+func TestTopicSkipsFullChannel(t *testing.T) {
 	topic := &app.Topic[int]{}
-	slow := make(chan *int) // unbuffered, will block
+	slow := make(chan *int) // unbuffered, so Send will skip this subscriber
+	fast := make(chan *int, 10)
 	topic.Watch(slow)
+	topic.Watch(fast)
 
 	val := 42
-	topic.Send(&val) // should drop the slow subscriber
+	topic.Send(&val) // slow is full (unbuffered), message dropped for slow only; fast receives
+	topic.Send(&val)
 
+	// Fast subscriber got both messages
+	for i := 0; i < 2; i++ {
+		select {
+		case got := <-fast:
+			if *got != 42 {
+				t.Fatalf("fast got %d", *got)
+			}
+		default:
+			t.Fatalf("expected fast to have message %d", i+1)
+		}
+	}
+	// Slow subscriber was skipped (channel full), not closed - channel still open and empty
+	select {
+	case _, ok := <-slow:
+		if !ok {
+			t.Fatal("slow channel should not be closed when subscriber is skipped")
+		}
+		t.Fatal("slow should have received no message")
+	default:
+		// slow is open and empty, which is correct
+	}
+	topic.Close()
 	_, ok := <-slow
 	if ok {
-		t.Fatal("expected slow subscriber channel to be closed")
+		t.Fatal("expected slow channel closed after topic.Close()")
 	}
 }