fix: use coverage circles instead of heatmap layer

Replace heatmap layer with concentric circle layers around scanner
positions. This avoids the z-index issue where heatmap appeared at
floor 0 below all markers.

Shows three coverage rings:
- Green inner (good signal ~5m)
- Yellow middle (fair signal ~10m)
- Red outer (weak signal ~15m)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/rf_mapper/web/static/js/app.js

@@ -2556,6 +2556,7 @@ function toggleHeatMap() {
 }
 
 // Render heat map layer on 3D map
+// Uses circle layers for scanner coverage visualization
 async function renderHeatMap() {
     if (!map3d || !map3dLoaded) {
         console.log('[HeatMap] 3D map not ready');
@@ -2572,12 +2573,15 @@ async function renderHeatMap() {
         // Remove existing heat map if present
         removeHeatMap();
 
-        // Add heat map source
+        // Filter to scanner positions only (weight=1.0 indicates scanners)
+        const scannerPoints = data.points.filter(p => p.weight >= 0.9);
+
+        // Add coverage circles source
         map3d.addSource('signal-heatmap', {
             type: 'geojson',
             data: {
                 type: 'FeatureCollection',
-                features: data.points.map(p => ({
+                features: scannerPoints.map(p => ({
                     type: 'Feature',
                     geometry: { type: 'Point', coordinates: [p.lon, p.lat] },
                     properties: { weight: p.weight }
@@ -2585,24 +2589,42 @@ async function renderHeatMap() {
             }
         });
 
-        // Add heat map layer
+        // Add outer coverage ring (weak signal ~15m)
         map3d.addLayer({
-            id: 'signal-heatmap-layer',
+            id: 'signal-heatmap-outer',
-            type: 'heatmap',
+            type: 'circle',
             source: 'signal-heatmap',
             paint: {
-                'heatmap-weight': ['get', 'weight'],
+                'circle-radius': 80,
-                'heatmap-intensity': 0.6,
+                'circle-color': 'rgba(255, 100, 100, 0.15)',
-                'heatmap-radius': 40,
+                'circle-stroke-width': 1,
-                'heatmap-opacity': 0.6,
+                'circle-stroke-color': 'rgba(255, 100, 100, 0.3)'
-                'heatmap-color': [
+            }
-                    'interpolate', ['linear'], ['heatmap-density'],
+        });
-                    0, 'rgba(0,0,255,0)',
+
-                    0.3, 'rgba(0,255,255,0.5)',
+        // Add middle coverage ring (fair signal ~10m)
-                    0.5, 'rgba(0,255,0,0.6)',
+        map3d.addLayer({
-                    0.7, 'rgba(255,255,0,0.7)',
+            id: 'signal-heatmap-middle',
-                    1, 'rgba(255,0,0,0.8)'
+            type: 'circle',
-                ]
+            source: 'signal-heatmap',
+            paint: {
+                'circle-radius': 50,
+                'circle-color': 'rgba(255, 255, 100, 0.2)',
+                'circle-stroke-width': 1,
+                'circle-stroke-color': 'rgba(255, 255, 100, 0.4)'
+            }
+        });
+
+        // Add inner coverage ring (good signal ~5m)
+        map3d.addLayer({
+            id: 'signal-heatmap-inner',
+            type: 'circle',
+            source: 'signal-heatmap',
+            paint: {
+                'circle-radius': 25,
+                'circle-color': 'rgba(100, 255, 100, 0.25)',
+                'circle-stroke-width': 1,
+                'circle-stroke-color': 'rgba(100, 255, 100, 0.5)'
             }
         });
     } catch (e) {
@@ -2614,9 +2636,13 @@ async function renderHeatMap() {
 function removeHeatMap() {
     if (!map3d) return;
 
-    if (map3d.getLayer('signal-heatmap-layer')) {
+    // Remove all heatmap layers
-        map3d.removeLayer('signal-heatmap-layer');
+    ['signal-heatmap-inner', 'signal-heatmap-middle', 'signal-heatmap-outer', 'signal-heatmap-layer'].forEach(layerId => {
-    }
+        if (map3d.getLayer(layerId)) {
+            map3d.removeLayer(layerId);
+        }
+    });
+
     if (map3d.getSource('signal-heatmap')) {
         map3d.removeSource('signal-heatmap');
     }