feat(rover): drop highlight marker on selected candidate

assets/js/rover_map_hook.ts

@@ -24,6 +24,7 @@ interface RoverMapHook extends ViewHook {
   homeMarker: L.CircleMarker | null
   driveCircle: L.Circle | null
   qualityLayer: L.GridLayer | null
+  selectedCandidateMarker: L.CircleMarker | null
   cellLookup: Map<string, Cell>
   homeLat: number
   homeLon: number
@@ -82,6 +83,7 @@ export const RoverMap: Partial<RoverMapHook> = {
     this.homeMarker = null
     this.driveCircle = null
     this.qualityLayer = null
+    this.selectedCandidateMarker = null
     this.cellLookup = new Map()
     this.topoLayer = null
 
@@ -210,6 +212,20 @@ export const RoverMap: Partial<RoverMapHook> = {
     this.handleEvent("focus_cell", (
       { lat, lon, zoom }: { lat: number; lon: number; zoom: number }
     ) => {
+      // Drop a single highlight marker at the picked spot, replacing
+      // any previous one — clicking another candidate moves it.
+      if (this.selectedCandidateMarker) {
+        this.map.removeLayer(this.selectedCandidateMarker)
+      }
+      this.selectedCandidateMarker = L.circleMarker([lat, lon], {
+        radius: 8,
+        color: "#0ea5e9",
+        weight: 2,
+        fillColor: "#0ea5e9",
+        fillOpacity: 0.6,
+        pane: "markerPane"
+      }).addTo(this.map)
+
       this.map.flyTo([lat, lon], zoom, { duration: 0.6 })
     })
   },