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/widgets/src/clide_column_hat.dart

@@ -33,7 +33,7 @@ class ColumnHat extends StatelessWidget {
   Widget build(BuildContext context) {
     final tokens = ClideTheme.of(context).surface;
     return GestureDetector(
-      onPanStart: (d) => windowControls.startDrag(d.globalPosition),
+      onPanStart: (_) => windowControls.startDrag(),
       child: Container(
         height: hatHeight,
         color: tokens.panelHeader,

lib/widgets/src/clide_resize_border.dart

@@ -45,7 +45,7 @@ class ClideResizeBorder extends StatelessWidget {
     return MouseRegion(
       cursor: cursor,
       child: GestureDetector(
-        onPanStart: (d) => windowControls.startResize(edge, d.globalPosition),
+        onPanStart: (_) => windowControls.startResize(edge),
         child: const ColoredBox(color: Color(0x00000000)),
       ),
     );