refactor(data): create layered architecture with core/data/ui packages

- Move constants from lib/config/ to lib/core/constants/
- Move color themes to lib/core/theme/
- Create LocationData domain model in data/models/
- Create lightweight GpsService in data/services/
- Create LocationRepository in data/repositories/ consolidating
  GpsDataManager business logic (GPS stream, staleness, precision)
- Create shared SpeedDisplay widget in ui/shared/
- Defer overlay feature — stubs reserved in ui/features/overlay/

lib/core/constants/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];
+  }
+}