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/overlay_constants.dart

@@ -34,4 +34,16 @@ abstract class OverlayConfig {
 
   /// Default overlay height for calculations
   static const double DEFAULT_OVERLAY_HEIGHT = 140.0;
+
+  /// Time after which overlay dims speed display due to stale data
+  static const Duration STALENESS_DIM_THRESHOLD = Duration(seconds: 3);
+
+  /// Time after which overlay shows dashes instead of speed
+  static const Duration STALENESS_DASH_THRESHOLD = Duration(seconds: 10);
+
+  /// Opacity value for dimmed stale state
+  static const double STALENESS_DIM_OPACITY = 0.5;
+
+  /// Interval for checking data staleness
+  static const Duration STALENESS_CHECK_INTERVAL = Duration(seconds: 1);
 }