fix(build): compile the handwriting demo as C++20

新加的 CI 在 windows-2025 上暴露出 msimeui-handwriting-demo 编译失败：MSVC 14.51 的 STL 对 <experimental/coroutine> 直接 static_assert 报错（STL1011）。

根因是这个 demo 用 C++/WinRT（winrt/Windows.UI.Input.Inking.h）做墨迹识别，而项目全局是 C++17；C++/WinRT 在 C++17 下走 <experimental/coroutine> 回退路径，新版 MSVC STL 已经不再允许。把这一个 target 提到 C++20 即可，库和其余三个 demo 不受影响。

ui/CMakeLists.txt

@@ -122,6 +122,10 @@ add_executable(msimeui-handwriting-demo WIN32
 
 target_link_libraries(msimeui-handwriting-demo PRIVATE msimeui windowsapp.lib)
 
+# C++/WinRT needs C++20 coroutines. Under C++17 it falls back to
+# <experimental/coroutine>, which recent MSVC STL versions reject outright.
+target_compile_features(msimeui-handwriting-demo PRIVATE cxx_std_20)
+
 if(MSVC)
     target_compile_options(msimeui PRIVATE /W4 /permissive-)
     target_compile_options(msimeui-demo PRIVATE /W4 /permissive-)