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.

frontend-nuxt/lib/SmartTileLayer.ts

@@ -58,7 +58,7 @@ export const SmartTileLayer = L.TileLayer.extend({
     return Util.template(this._url, Util.extend(data, this.options))
   },
 
-  refresh(x: number, y: number, z: number) {
+  refresh(x: number, y: number, z: number): boolean {
     let zoom = z
     const maxZoom = this.options.maxZoom
     const zoomReverse = this.options.zoomReverse
@@ -71,19 +71,20 @@ export const SmartTileLayer = L.TileLayer.extend({
 
     const key = `${x}:${y}:${zoom}`
     const tile = this._tiles[key]
-    if (!tile?.el) return
+    if (!tile?.el) return false
     const newUrl = this.getTrueTileUrl({ x, y }, z)
-    if (tile.el.dataset.tileUrl === newUrl) return
+    if (tile.el.dataset.tileUrl === newUrl) return true
     tile.el.dataset.tileUrl = newUrl
     tile.el.src = newUrl
     tile.el.classList.add('tile-fresh')
     const el = tile.el
     setTimeout(() => el.classList.remove('tile-fresh'), 400)
+    return true
   },
 }) as unknown as new (urlTemplate: string, options?: L.TileLayerOptions) => L.TileLayer & {
   cache: SmartTileLayerCache
   invalidTile: string
   map: number
   getTrueTileUrl: (coords: { x: number; y: number }, zoom: number) => string
-  refresh: (x: number, y: number, z: number) => void
+  refresh: (x: number, y: number, z: number) => boolean
 }