Wrap a few things trying to prevent UI / main thread hang we saw one time

lib/services/map_data_submodules/nodes_from_overpass.dart

@@ -150,7 +150,7 @@ Future<List<OsmNode>> _fetchSingleOverpassQuery({
       return [];
     }
     
-    final data = jsonDecode(response.body) as Map<String, dynamic>;
+    final data = await compute(jsonDecode, response.body) as Map<String, dynamic>;
     final elements = data['elements'] as List<dynamic>;
     
     if (elements.length > 20) {

lib/services/node_cache.dart

@@ -11,6 +11,7 @@ class NodeCache {
   final Map<int, OsmNode> _nodes = {};
 
   /// Add or update a batch of nodes in the cache.
+  /// TODO: Consider moving to compute() if cache operations cause ANR
   void addOrUpdate(List<OsmNode> nodes) {
     for (var node in nodes) {
       final existing = _nodes[node.id];

lib/services/suspected_location_service.dart

@@ -120,11 +120,7 @@ class SuspectedLocationService {
       onProgress?.call('Parsing CSV data...', 0.2);
       
       // Parse CSV with proper field separator and quote handling
-      final csvData = const CsvToListConverter(
-        fieldDelimiter: ',',
-        textDelimiter: '"',
-        eol: '\n',
-      ).convert(response.body);
+      final csvData = await compute(_parseCSV, response.body);
       debugPrint('[SuspectedLocationService] Parsed ${csvData.length} rows from CSV');
       
       if (csvData.isEmpty) {
@@ -223,4 +219,13 @@ class SuspectedLocationService {
       LatLng(south, east),
     ));
   }
+}
+
+/// Simple CSV parser for compute() - must be top-level function
+List<List<dynamic>> _parseCSV(String csvBody) {
+  return const CsvToListConverter(
+    fieldDelimiter: ',',
+    textDelimiter: '"',
+    eol: '\n',
+  ).convert(csvBody);
 }