fix mouse jumping: query GDK pointer position at call time

Flutter's globalPosition is window-relative, not screen-absolute.
Now using gdk_device_get_position() on the native side to get
the actual screen coordinates at the moment startDrag/startResize
is called. Dart no longer passes coordinates — the native handler
queries the pointer directly.

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

lib/app.dart

@@ -221,7 +221,7 @@ class _HatBar extends StatelessWidget {
   Widget build(BuildContext context) {
     final tokens = ClideTheme.of(context).surface;
     return GestureDetector(
-      onPanStart: (d) => kernel.window.startDrag(d.globalPosition),
+      onPanStart: (_) => kernel.window.startDrag(),
       child: Container(
         height: hatHeight,
         decoration: BoxDecoration(