feat(01-01): extract models and move config files to new structure

- Move SpeedUnit enum to config/speed_unit.dart
- Move ColorTheme/ColorThemes classes to config/color_themes.dart
- Extract ProcessedGpsData model from gps_data_manager to models/
- Create OverlayMessage typed class for overlay IPC (replaces raw Maps)
- Create ThemeModel with palette rotation and state management

lib/models/theme_model.dart

@@ -0,0 +1,32 @@
+import '../config/color_themes.dart';
+
+/// Theme model that wraps ColorThemes with rotation and state management.
+/// Holds all possible color palettes via delegation, exposing only the current set.
+class ThemeModel {
+  final int _currentIndex;
+
+  const ThemeModel({int currentIndex = 0}) : _currentIndex = currentIndex;
+
+  /// Get the current active color theme
+  ColorTheme get current => ColorThemes.getTheme(_currentIndex);
+
+  /// Get the name of the current theme
+  String get currentSetName => current.name;
+
+  /// Get the current theme index
+  int get currentIndex => _currentIndex;
+
+  /// Rotate to the next theme (circular)
+  ThemeModel rotate() {
+    return ThemeModel(
+      currentIndex: ColorThemes.getNextThemeIndex(_currentIndex),
+    );
+  }
+
+  /// Create a copy with an updated theme index
+  ThemeModel copyWith({int? currentIndex}) {
+    return ThemeModel(
+      currentIndex: currentIndex ?? _currentIndex,
+    );
+  }
+}