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/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: (_) => windowControls.startDrag(),
+      onPanStart: (d) => windowControls.startDrag(d.globalPosition),
       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: (_) => windowControls.startResize(edge),
+        onPanStart: (d) => windowControls.startResize(edge, d.globalPosition),
         child: const ColoredBox(color: Color(0x00000000)),
       ),
     );