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/main.dart

@@ -1,13 +1,19 @@
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import 'package:shared_preferences/shared_preferences.dart';
-import 'screens/speedometer_screen.dart';
-import 'screens/overlay_screen.dart';
-import 'services/gps_data_manager.dart';
-import 'services/overlay_service.dart';
+import 'ui/features/speedometer/speedometer_view.dart';
+import 'ui/features/speedometer/speedometer_view_model.dart';
 import 'providers/settings_provider.dart';
-import 'providers/overlay_provider.dart';
-import 'services/logger.dart';
+import 'data/repositories/location_repository.dart';
+
+/// Stub ViewModel for the overlay feature.
+///
+/// [Overlay] has been deferred — see ui/features/overlay/ for details.
+class OverlayViewModel extends ChangeNotifier {
+  bool get isOverlayActive => false;
+  void toggleOverlay() {}
+  void closeOverlay() {}
+}
 
 void main() async {
   WidgetsFlutterBinding.ensureInitialized();
@@ -16,22 +22,29 @@ void main() async {
   runApp(
     MultiProvider(
       providers: [
+        // Shared
         Provider<SharedPreferences>.value(value: prefs),
-        Provider<OverlayService>(create: (_) => OverlayService()),
-        ChangeNotifierProvider(
-          create: (context) => SettingsProvider(context.read<SharedPreferences>()),
+        // Data layer
+        ChangeNotifierProvider<LocationRepository>(
+          create: (_) {
+            final repo = LocationRepository();
+            repo.initialize();
+            return repo;
+          },
         ),
-        ChangeNotifierProvider(
-          create: (_) => GpsDataManager(),
+        // UI layer — ViewModels subscribe to repositories
+        ChangeNotifierProvider<SpeedometerViewModel>(
+          create: (context) => SpeedometerViewModel(
+            context.read<LocationRepository>(),
+          ),
         ),
-        ChangeNotifierProxyProvider2<GpsDataManager, SettingsProvider, OverlayProvider>(
-          create: (context) => OverlayProvider(context.read<OverlayService>()),
-          update: (_, gpsManager, settings, overlay) => overlay!
-            ..updateDependencies(
-              gpsManager: gpsManager,
-              currentUnit: settings.currentUnit,
-              currentThemeIndex: settings.currentThemeIndex,
-            ),
+        ChangeNotifierProvider<OverlayViewModel>(
+          create: (_) => OverlayViewModel(),
+        ),
+        ChangeNotifierProvider<SettingsProvider>(
+          create: (context) => SettingsProvider(
+            context.read<SharedPreferences>(),
+          ),
         ),
       ],
       child: const SpeedoApp(),
@@ -39,15 +52,6 @@ void main() async {
   );
 }
 
-@pragma("vm:entry-point")
-void overlayMain() {
-  Logger.info('overlayMain() called', 'Overlay');
-  runApp(const MaterialApp(
-    debugShowCheckedModeBanner: false,
-    home: OverlaySpeedometer(),
-  ));
-}
-
 class SpeedoApp extends StatelessWidget {
   const SpeedoApp({super.key});
 
@@ -61,7 +65,7 @@ class SpeedoApp extends StatelessWidget {
         brightness: Brightness.dark,
         colorSchemeSeed: Colors.green,
       ),
-      home: const SpeedometerScreen(),
+      home: const SpeedometerView(),
     );
   }
-}
+}