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/processed_gps_data.dart

@@ -0,0 +1,35 @@
+class ProcessedGpsData {
+  final double speed;           // Raw speed in m/s
+  final double heading;         // Raw heading in degrees
+  final String displaySpeed;    // Formatted speed text (with -- logic)
+  final String displayHeading;  // Formatted heading text
+  final bool isSpeedValid;      // True if from recent GPS
+  final bool isHeadingValid;    // True if from recent GPS
+
+  const ProcessedGpsData({
+    required this.speed,
+    required this.heading,
+    required this.displaySpeed,
+    required this.displayHeading,
+    required this.isSpeedValid,
+    required this.isHeadingValid,
+  });
+
+  ProcessedGpsData copyWith({
+    double? speed,
+    double? heading,
+    String? displaySpeed,
+    String? displayHeading,
+    bool? isSpeedValid,
+    bool? isHeadingValid,
+  }) {
+    return ProcessedGpsData(
+      speed: speed ?? this.speed,
+      heading: heading ?? this.heading,
+      displaySpeed: displaySpeed ?? this.displaySpeed,
+      displayHeading: displayHeading ?? this.displayHeading,
+      isSpeedValid: isSpeedValid ?? this.isSpeedValid,
+      isHeadingValid: isHeadingValid ?? this.isHeadingValid,
+    );
+  }
+}