lte service (backend) and ui handling

pi/ui/lib/services/backend_service.dart

@@ -58,6 +58,25 @@ class GpsData {
   }
 }
 
+/// Data from LTE modem (signal quality, connection state)
+class LteData {
+  final bool? connected;
+  final int? signal; // 0-100 percent
+  final String? operator_;
+  final String? accessTech;
+
+  LteData({this.connected, this.signal, this.operator_, this.accessTech});
+
+  factory LteData.fromJson(Map<String, dynamic> json) {
+    return LteData(
+      connected: json['connected'] as bool?,
+      signal: (json['signal'] as num?)?.toInt(),
+      operator_: json['operator'] as String?,
+      accessTech: json['access_tech'] as String?,
+    );
+  }
+}
+
 /// HTTP client for Flask backend - fire-and-forget async fetch, sync cache return
 ///
 /// Follows the same pattern as PiIO: never blocks UI, always returns cached data.