feat(03-01): create OverlayService and add retry dependency

- Add retry package (^3.1.2) for robust platform operations
- Create OverlayService wrapper for all FlutterOverlayWindow calls
- Implement showOverlay/closeOverlay with 3-attempt retry logic
- Add shareData fire-and-forget with warn-level logging
- Add isActive, overlayListener, checkPermission helpers
- Define staleness constants: 3s dim, 10s dash, 0.5 opacity, 1s check
- Define GPS_GRACE_PERIOD constant (30 seconds)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/config/timing_constants.dart

@@ -13,4 +13,7 @@ abstract class TimingConfig {
 
   /// 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
+  static const Duration GPS_GRACE_PERIOD = Duration(seconds: 30);
 }