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>

pubspec.yaml

@@ -33,6 +33,9 @@ dependencies:
   # Persistent settings storage
   shared_preferences: ^2.3.4
 
+  # Retry logic for platform operations
+  retry: ^3.1.2
+
 
 dev_dependencies:
   flutter_test: