feat: Add Meshtastic, Ubertooth, and Offline Mode support

New Features:
- Meshtastic LoRa mesh network integration
  - Real-time message streaming via SSE
  - Channel configuration with encryption
  - Node information with RSSI/SNR metrics
- Ubertooth One BLE scanner backend
  - Passive capture across all 40 BLE channels
  - Raw advertising payload access
- Offline mode with bundled assets
  - Local Leaflet, Chart.js, and fonts
  - Multiple map tile providers
  - Settings modal for configuration

Technical Changes:
- New routes: meshtastic.py, offline.py
- New utils: ubertooth_scanner.py, meshtastic.py
- New CSS/JS for meshtastic and settings
- Updated dashboard templates with conditional asset loading
- Added context processor for offline settings

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

utils/bluetooth/fallback_scanner.py

@@ -531,6 +531,16 @@ class FallbackScanner:
         except Exception:
             pass
 
+        # Try ubertooth (raw packet capture with Ubertooth One hardware)
+        try:
+            from .ubertooth_scanner import UbertoothScanner
+            self._active_scanner = UbertoothScanner(on_observation=self._on_observation)
+            if self._active_scanner.start():
+                self._backend = 'ubertooth'
+                return True
+        except Exception:
+            pass
+
         logger.error("No fallback scanner available")
         return False