Add flutter_lints and fix analyzer errors, dead code, and unused imports

lib/services/map_data_submodules/nodes_from_osm_api.dart

@@ -1,4 +1,3 @@
-import 'dart:convert';
 import 'package:http/http.dart' as http;
 import 'package:flutter/foundation.dart';
 import 'package:latlong2/latlong.dart';
@@ -81,7 +80,7 @@ Future<List<OsmNode>> _fetchFromOsmApi({
     debugPrint('[fetchOsmApiNodes] Exception: $e');
     
     // Don't report status here - let the top level handle it
-    throw e; // Re-throw to let caller handle
+    rethrow; // Re-throw to let caller handle
   }
 }
 

lib/services/map_data_submodules/tiles_from_local.dart

@@ -1,5 +1,4 @@
 import 'dart:io';
-import 'package:latlong2/latlong.dart';
 import '../offline_area_service.dart';
 import '../offline_areas/offline_area_models.dart';
 import '../offline_areas/offline_tile_utils.dart';