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/timing_constants.dart

@@ -0,0 +1,23 @@
+// Timer and interval constants — SCREAMING_SNAKE per project convention
+
+/// Timing configuration for background tasks and UI interactions
+abstract class TimingConfig {
+  /// Interval for background heartbeat to keep GPS alive
+  static const Duration HEARTBEAT_INTERVAL = Duration(seconds: 5);
+
+  /// Interval for checking overlay window status
+  static const Duration OVERLAY_STATUS_CHECK_INTERVAL = Duration(milliseconds: 1000);
+
+  /// Delay before closing overlay after tap
+  static const Duration TAP_CLOSE_DELAY = Duration(milliseconds: 500);
+
+  /// Delay for long press gesture signal
+  static const Duration LONG_PRESS_SIGNAL_DELAY = Duration(milliseconds: 50);
+
+  /// GPS grace period - continues for 30s after overlay close when backgrounded
+  /// DEPRECATED: Replaced by BACKGROUND_GRACE_PERIOD (7s) for lifecycle management
+  static const Duration GPS_GRACE_PERIOD = Duration(seconds: 30);
+
+  /// Background grace period - time before GPS stops when app backgrounded without overlay
+  static const Duration BACKGROUND_GRACE_PERIOD = Duration(seconds: 7);
+}