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/handlers/tile.go

@@ -61,8 +61,17 @@ func (h *Handlers) WatchGridUpdates(rw http.ResponseWriter, req *http.Request) {
 
 	ticker := time.NewTicker(app.SSETickInterval)
 	defer ticker.Stop()
+	keepaliveTicker := time.NewTicker(app.SSEKeepaliveInterval)
+	defer keepaliveTicker.Stop()
 	for {
 		select {
+		case <-ctx.Done():
+			return
+		case <-keepaliveTicker.C:
+			if _, err := fmt.Fprint(rw, ": keepalive\n\n"); err != nil {
+				return
+			}
+			flusher.Flush()
 		case e, ok := <-c:
 			if !ok {
 				return
@@ -99,7 +108,9 @@ func (h *Handlers) WatchGridUpdates(rw http.ResponseWriter, req *http.Request) {
 		case <-ticker.C:
 			raw, _ := json.Marshal(tileCache)
 			fmt.Fprint(rw, "data: ")
-			_, _ = rw.Write(raw)
+			if _, err := rw.Write(raw); err != nil {
+				return
+			}
 			fmt.Fprint(rw, "\n\n")
 			tileCache = tileCache[:0]
 			flusher.Flush()