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>
jpmschweitzerandClaude Opus 4.6
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