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/gps_constants.dart

@@ -0,0 +1,31 @@
+// GPS behavioral constants — SCREAMING_SNAKE per project convention
+
+/// GPS configuration constants for data validation and timeouts
+abstract class GpsConfig {
+  /// Time threshold after which GPS data is considered stale
+  static const Duration STALE_DATA_THRESHOLD = Duration(seconds: 4);
+
+  /// Timeout for initial GPS fix acquisition (outdoor/cold start)
+  static const Duration INITIAL_FIX_TIMEOUT = Duration(seconds: 20);
+
+  /// Timeout for ongoing GPS updates (indoor/signal loss detection)
+  static const Duration UPDATE_TIMEOUT = Duration(seconds: 2);
+
+  /// Minimum valid heading value in degrees
+  static const double VALID_HEADING_MIN = 0.0;
+
+  /// Maximum valid heading value in degrees
+  static const double VALID_HEADING_MAX = 360.0;
+
+  /// Speed threshold below which heading is considered unreliable (m/s)
+  static const double LOW_SPEED_THRESHOLD = 1.0;
+
+  /// Speed threshold for switching to HIGH precision (m/s) - 12 km/h
+  static const double PRECISION_UP_THRESHOLD_MPS = 3.33;
+
+  /// Speed threshold for switching to BALANCED precision (m/s) - 8 km/h
+  static const double PRECISION_DOWN_THRESHOLD_MPS = 2.22;
+
+  /// Minimum time after first fix before precision switching starts
+  static const Duration PRECISION_SWITCH_FIRST_FIX_DELAY = Duration(seconds: 5);
+}