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/providers/settings_provider.dart

@@ -1,8 +1,8 @@
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 import 'package:shared_preferences/shared_preferences.dart';
-import '../config/speed_unit.dart';
-import '../config/color_themes.dart';
+import '../core/constants/speed_unit.dart';
+import '../core/theme/color_themes.dart';
 import '../services/logger.dart';
 
 class SettingsProvider extends ChangeNotifier {