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>

linux/runner/CMakeLists.txt

@@ -1,26 +1,27 @@
 cmake_minimum_required(VERSION 3.13)
-project(runner LANGUAGES CXX)
+project(runner LANGUAGES C CXX)
 
-# Define the application target. To change its name, change BINARY_NAME in the
-# top-level CMakeLists.txt, not the value here, or `flutter run` will no longer
-# work.
-#
 # Any new source files that you add to the application should be added here.
 add_executable(${BINARY_NAME}
   "main.cc"
   "my_application.cc"
+  "protocols/xdg-decoration-unstable-v1-protocol.c"
   "${FLUTTER_MANAGED_DIR}/generated_plugin_registrant.cc"
 )
 
-# Apply the standard set of build settings. This can be removed for applications
-# that need different build settings.
 apply_standard_settings(${BINARY_NAME})
 
-# Add preprocessor definitions for the application ID.
 add_definitions(-DAPPLICATION_ID="${APPLICATION_ID}")
 
-# Add dependency libraries. Add any application-specific dependencies here.
+# Find wayland-client for xdg-decoration protocol (D-057).
+pkg_check_modules(WAYLAND_CLIENT IMPORTED_TARGET wayland-client)
+
 target_link_libraries(${BINARY_NAME} PRIVATE flutter)
 target_link_libraries(${BINARY_NAME} PRIVATE PkgConfig::GTK)
+if(WAYLAND_CLIENT_FOUND)
+  target_link_libraries(${BINARY_NAME} PRIVATE PkgConfig::WAYLAND_CLIENT)
+  target_compile_definitions(${BINARY_NAME} PRIVATE HAS_WAYLAND_CLIENT=1)
+endif()
 
 target_include_directories(${BINARY_NAME} PRIVATE "${CMAKE_SOURCE_DIR}")
+target_include_directories(${BINARY_NAME} PRIVATE "${CMAKE_SOURCE_DIR}/runner")