implement xdg-decoration for native Wayland frameless

Replace XWayland fallback with proper xdg-decoration protocol
negotiation. On Wayland: binds zxdg_decoration_manager_v1 from
the compositor registry, requests CLIENT_SIDE mode on the toplevel
surface — KWin respects this and drops server-side decorations.
Falls back to gdk_window_set_decorations(0) + set_decorated(FALSE)
on X11. GDK_BACKEND=x11 removed from Makefile.

Also fixes mouse-jumping glitch: startDrag and startResize now
pass screen coordinates from Flutter's globalPosition through the
method channel to gtk_window_begin_move/resize_drag.

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

lib/kernel/src/window_controls.dart

@@ -17,19 +17,26 @@ class WindowControls extends ChangeNotifier {
     notifyListeners();
   }
 
-  Future<void> startResize(ResizeEdge edge) async {
+  Future<void> startResize(ResizeEdge edge, Offset screenPosition) async {
     try {
-      await _channel.invokeMethod('startResize', edge.index);
+      await _channel.invokeMethod('startResize', {
+        'edge': edge.index,
+        'x': screenPosition.dx.round(),
+        'y': screenPosition.dy.round(),
+      });
     } on MissingPluginException {
       // no-op
     }
   }
 
-  Future<void> startDrag() async {
+  Future<void> startDrag(Offset screenPosition) async {
     try {
-      await _channel.invokeMethod('startDrag');
+      await _channel.invokeMethod('startDrag', {
+        'x': screenPosition.dx.round(),
+        'y': screenPosition.dy.round(),
+      });
     } on MissingPluginException {
-      // Web or unsupported platform — no-op.
+      // no-op
     }
   }