use KDE server-decoration protocol for native Wayland frameless

Replace xdg-decoration (needs xdg_toplevel, GTK3 doesn't expose it)
with org_kde_kwin_server_decoration (works with wl_surface which
GDK provides). On KDE Wayland: binds the manager from the registry,
creates a per-surface decoration object, requests mode=None (0).
Falls back gracefully on GNOME (protocol not present, GTK's
set_decorated(FALSE) already works). X11 fallback via
gdk_window_set_decorations(0) unchanged.

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

linux/runner/CMakeLists.txt

@@ -6,6 +6,7 @@ add_executable(${BINARY_NAME}
   "main.cc"
   "my_application.cc"
   "protocols/xdg-decoration-unstable-v1-protocol.c"
+  "protocols/server-decoration-protocol.c"
   "${FLUTTER_MANAGED_DIR}/generated_plugin_registrant.cc"
 )