widen drag handle to 8px hit area

Still 1px visible line; 8px aligns with Flutter's 8-point grid.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/kernel/src/panels/drag_resize.dart

@@ -20,7 +20,7 @@ class DragResizeHandle extends StatefulWidget {
   final Axis axis;
   final double thickness;
 
-  static const defaultThickness = 5.0;
+  static const defaultThickness = 8.0;
 
   @override
   State<DragResizeHandle> createState() => _DragResizeHandleState();