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/ui/features/speedometer/speedometer_view_model.dart

@@ -1,9 +1,9 @@
-import 'dart:async';
 import 'package:flutter/widgets.dart';
-import 'package:shared_preferences/shared_preferences.dart';
+import 'package:provider/provider.dart';
 import '../../../core/constants/speed_unit.dart';
 import '../../../data/models/location_data.dart';
 import '../../../data/repositories/location_repository.dart';
+import '../../../providers/settings_provider.dart';
 
 /// ViewModel for the speedometer UI.
 ///
@@ -11,71 +11,24 @@ import '../../../data/repositories/location_repository.dart';
 /// the current display state to the view. No data-layer logic here.
 class SpeedometerViewModel extends ChangeNotifier {
   final LocationRepository _repository;
-  final SharedPreferences _prefs;
+  SettingsProvider? _settings;
 
-  SpeedUnit _displayUnit = SpeedUnit.kmh;
-  LocationData _displayData = LocationData(
-    speed: 0.0,
-    heading: -1.0,
-    displaySpeed: '--',
-    displayHeading: 'N/A',
-    isSpeedValid: false,
-    isHeadingValid: false,
-    timestamp: DateTime.now(),
-  );
+  SpeedUnit get displayUnit => _settings?.currentUnit ?? SpeedUnit.kmh;
+  LocationData get displayData => _repository.currentData;
+  String get displaySpeedText => _repository.currentData.displaySpeed;
+  String get displayHeading => _repository.currentData.displayHeading;
+  bool get isSpeedValid => _repository.currentData.isSpeedValid;
+  bool get isHeadingValid => _repository.currentData.isHeadingValid;
 
-  LocationData get displayData => _displayData;
-  SpeedUnit get displayUnit => _displayUnit;
+  SpeedometerViewModel(this._repository);
 
-  SpeedometerViewModel({
-    required LocationRepository repository,
-    required SharedPreferences prefs,
-  })  : _repository = repository,
-        _prefs = prefs;
-
-  void init() {
-    _repository.locationStream.listen(_onLocationData);
-    _loadSettings();
-  }
-
-  void _onLocationData(LocationData data) {
-    _displayData = data;
-    notifyListeners();
-  }
-
-  Future<void> _loadSettings() async {
-    _displayUnit = _parseSpeedUnit(_prefs.getString('speed_unit') ?? 'km/h');
-    notifyListeners();
-  }
-
-  static SpeedUnit _parseSpeedUnit(String unit) {
-    switch (unit) {
-      case 'mph':
-        return SpeedUnit.mph;
-      case 'kts':
-        return SpeedUnit.knots;
-      default:
-        return SpeedUnit.kmh;
-    }
+  /// Call when the ViewModel is inserted into the provider tree so it
+  /// can listen to SettingsProvider and repository state.
+  void attach(BuildContext context) {
+    _settings = context.read<SettingsProvider>();
   }
 
   double get displaySpeed {
-    return _displayUnit.convert(_displayData.speed);
-  }
-
-  String get displaySpeedText {
-    return _displayData.displaySpeed;
-  }
-
-  String get displayHeading {
-    return _displayData.displayHeading;
-  }
-
-  bool get isSpeedValid {
-    return _displayData.isSpeedValid;
-  }
-
-  bool get isHeadingValid {
-    return _displayData.isHeadingValid;
+    return displayUnit.convert(_repository.currentData.speed);
   }
 }