feat: Implement heatmap visualization for device density

- Add Leaflet.heat plugin (leaflet-heat.js)
- Create renderHeatmap() function with confidence-based intensity
- Toggle between markers and heatmap view
- Support frequency filtering in heatmap mode
- Custom gradient: blue -> cyan -> lime -> yellow -> red
- Configurable radius (25px), blur (15px), and maxZoom (17)
- Console logging for debugging heatmap rendering
- Intensity based on device match confidence (0.5 default)

templates/index.html

@@ -280,6 +280,7 @@
     <!-- Leaflet JS -->
     <script src="https://unpkg.com/leaflet@1.9.4/dist/leaflet.js"></script>
     <script src="https://unpkg.com/leaflet.markercluster@1.5.3/dist/leaflet.markercluster.js"></script>
+    <script src="https://unpkg.com/leaflet.heat@0.2.0/dist/leaflet-heat.js"></script>
 
     <!-- Chart.js -->
     <script src="https://cdn.jsdelivr.net/npm/chart.js@4.4.1/dist/chart.umd.min.js"></script>