Fix contribute watchOngoingGameInFile for non-ASCII paths on Windows

[Hoigin]

Summary

On Windows, if the path in watchOngoingGameFileName contains non-ASCII characters, like D:\Go\跑谱\watchgame.txt, KataGo fails to open the watch file, and the whole contribute process terminates a few seconds after startup with:

ERROR: game loop loop thread failed: Could not open file: D:\Go\跑谱\watchgame.txt

The root cause is that the Windows branch opens the file with the narrow-char fopen_s, which interprets the UTF-8 path bytes in the ANSI codepage, so the path no longer matches the real directory (a TODO in contribute.cpp already notes that non-ascii paths are unhandled).

This PR converts the path to wide chars and opens it with _wfopen, the same pattern already used in homedata.cpp and mainargs.cpp.

Testing

Verified on Windows 11 (Chinese locale, GBK codepage), KataGo v1.18.2 + this change, MSVC build with -DBUILD_DISTRIBUTED=1:

• A path containing a non-ASCII directory (D:\Go\跑谱\watchgame.txt) now creates the file at the correct path. ASCII-only paths behave as before.
• Invalid UTF-8 in the config value produces a clear StringError.

[lightvector]

Thanks for reporting! Fixed in a slightly different way, in 9bb7efc

[Hoigin]

Verified the alternative fix, closing this in favor of it. Thanks for the great project!