refactor: complete layered architecture migration

Delete old flat files, wire new DI in main.dart:
- Removed: gps_service.dart, config/, models/, providers/overlay_provider.dart,
  screens/, services/gps_data_manager.dart, services/overlay_service.dart
- Kept: settings_provider.dart (thin, correct), logger.dart (shared util)
- main.dart now wires: LocationRepository → SpeedometerViewModel → SpeedometerView
- overlay_provider.dart deferred, stub in ui/features/overlay/
- flutter analyze: 30 info-level constant_identifier_names warnings (project convention),
  zero errors, zero warnings — builds clean

New structure:
  lib/
    core/constants/     — gps, overlay, timing, speed_unit
    core/theme/         — color_themes
    data/
      models/           — LocationData (immutable domain model)
      services/         — GpsService (stateless wrapper), OverlayService (IPC)
      repositories/     — LocationRepository (business logic, ChangeNotifier)
    providers/          — SettingsProvider (thin, unchanged)
    ui/
      features/
        speedometer/    — SpeedometerViewModel + SpeedometerView
        overlay/        — Stub ViewModel + View (deferred)
      shared/           — SpeedDisplay widget
    services/           — Logger (shared util)

lib/data/repositories/location_repository.dart

@@ -13,7 +13,7 @@ import '../services/gps_service.dart';
 /// - GPS stream management (start/stop/restart)
 /// - Data processing (staleness, precision switching, validation)
 /// - Lifecycle management (app foreground/background, wake lock)
-class LocationRepository extends WidgetsBindingObserver {
+class LocationRepository extends ChangeNotifier with WidgetsBindingObserver {
   StreamSubscription<Position>? _gpsSubscription;
   final StreamController<LocationData> _dataController =
       StreamController<LocationData>.broadcast();
@@ -238,6 +238,7 @@ class LocationRepository extends WidgetsBindingObserver {
     if (_isDisposed) return;
     _currentData = newData;
     _dataController.add(newData);
+    notifyListeners();
     Logger.debug('Broadcasting data: ${newData.displaySpeed} ${newData.displayHeading}', 'LocationRepository');
   }
 
@@ -315,7 +316,9 @@ class LocationRepository extends WidgetsBindingObserver {
     );
   }
 
+  @override
   void dispose() {
+    super.dispose();
     _isDisposed = true;
     Logger.info('Disposing location repository...', 'LocationRepository');
     WidgetsBinding.instance.removeObserver(this);