Add Leaflet.heat crowd density heatmap to GSM Spy dashboard

Adds a toggleable heatmap layer that visualizes crowd density data from
the existing /gsm_spy/crowd_density endpoint as a gradient overlay on the
map, with auto-refresh every 30s during active monitoring.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

routes/offline.py

@@ -13,7 +13,7 @@ OFFLINE_DEFAULTS = {
     'offline.enabled': False,
     'offline.assets_source': 'cdn',
     'offline.fonts_source': 'cdn',
-    'offline.tile_provider': 'cartodb_dark_cyan',
+    'offline.tile_provider': 'cartodb_dark_cyan',
     'offline.tile_server_url': ''
 }
 
@@ -44,6 +44,9 @@ ASSET_PATHS = {
         'static/vendor/leaflet/images/marker-shadow.png',
         'static/vendor/leaflet/images/layers.png',
         'static/vendor/leaflet/images/layers-2x.png'
+    ],
+    'leaflet_heat': [
+        'static/vendor/leaflet-heat/leaflet-heat.js'
     ]
 }