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/config/speed_unit.dart

@@ -0,0 +1,20 @@
+enum SpeedUnit {
+  kmh('km/h', 1.0),
+  mph('mph', 0.621371),
+  knots('kts', 0.539957);
+
+  const SpeedUnit(this.label, this.multiplier);
+
+  final String label;
+  final double multiplier;
+
+  double convert(double speedMps) {
+    return speedMps * 3.6 * multiplier;
+  }
+
+  SpeedUnit get next {
+    final values = SpeedUnit.values;
+    final currentIndex = values.indexOf(this);
+    return values[(currentIndex + 1) % values.length];
+  }
+}