fix: correct location validation and clean up target contact handling

- Fix asymmetric lat/lon validation in _handleContactAdvert (was checking
  longitude != 0 for latitude; now uses (latitude != 0 || longitude != 0)
  for both)
- Remove duplicate targetGuessed assignment in path_trace_map
- Rename public target field to private _targetContact, use local variable
  to avoid unnecessary null-aware operators

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/connector/meshcore_connector.dart

@@ -4757,14 +4757,14 @@ class MeshCoreConnector extends ChangeNotifier {
             hasLocation &&
                 latitude != null &&
                 latitude.abs() <= 90 &&
-                longitude != 0
+                (latitude != 0 || longitude != 0)
             ? latitude
             : existing.latitude,
         longitude:
             hasLocation &&
                 longitude != null &&
                 longitude.abs() <= 180 &&
-                longitude != 0
+                (latitude != 0 || longitude != 0)
             ? longitude
             : existing.longitude,
         name: hasName ? name : existing.name,