remove xdg-decoration leftovers, add make build-check

The xdg-decoration protocol files caused a linker error
(xdg_toplevel_interface undefined) since we switched to the KDE
server-decoration protocol. Removed. Added make build-check
target that runs flutter build linux to catch native compilation
failures that flutter test misses.

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

linux/runner/CMakeLists.txt

@@ -5,7 +5,6 @@ project(runner LANGUAGES C CXX)
 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"
 )