Address PR review: truncate error response logs and close http client

- Gate full error response body logging behind kDebugMode; truncate to
  500 chars in release builds to avoid log noise and data exposure
- Add RoutingService.close() and call from NavigationState.dispose()

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/state/navigation_state.dart

@@ -376,4 +376,10 @@ class NavigationState extends ChangeNotifier {
       notifyListeners();
     }
   }
+
+  @override
+  void dispose() {
+    _routingService.close();
+    super.dispose();
+  }
 }