refactor(01-01): rewire all imports, replace logging, apply constants

- Update gps_data_manager.dart: import ProcessedGpsData from models/, use GpsConfig constants, replace all print() with Logger calls
- Update gps_service.dart: use GpsConfig.VALID_HEADING_MIN/MAX for heading validation
- Update main.dart: import from config/, models/, replace all customDebugPrint() and print() with Logger, use TimingConfig/OverlayConfig constants, convert overlay IPC to OverlayMessage typed class
- Delete customDebugPrint() function definition
- Delete lib/speed_units.dart and lib/color_themes.dart (moved to config/)
- Zero analyzer errors, zero bare print() calls remaining

lib/gps_service.dart

@@ -1,10 +1,11 @@
 import 'dart:async';
 import 'package:geolocator/geolocator.dart';
 import 'package:permission_handler/permission_handler.dart';
+import 'config/gps_constants.dart';
 
 class GpsService {
   static double? _lastValidHeading;
-  
+
   static Stream<Position> get positionStream {
     return Geolocator.getPositionStream(
       locationSettings: const LocationSettings(
@@ -13,18 +14,18 @@ class GpsService {
       ),
     );
   }
-  
+
   static Future<bool> requestPermissions() async {
     final permission = await Permission.locationWhenInUse.request();
     return permission.isGranted;
   }
-  
+
   static Future<bool> isLocationServiceEnabled() async {
     return await Geolocator.isLocationServiceEnabled();
   }
-  
+
   static double getCachedHeading(double heading) {
-    if (heading >= 0 && heading <= 360) {
+    if (heading >= GpsConfig.VALID_HEADING_MIN && heading <= GpsConfig.VALID_HEADING_MAX) {
       _lastValidHeading = heading;
       return heading;
     }