[gui] Fix non-ASCII filename rendering in TRootBrowser

[wacfrr]

ROOT's application manifest explicitly declares UTF-8 active code page, so strings entering the GDK backend are guaranteed to be valid UTF-8. Use explicit UTF-8 decoding instead of mbstowcs(), which would misinterpret the UTF-8 byte stream under the system's legacy ANSI code page.

This Pull request:

Fixes non-ASCII (CJK and other) filename display in TRootBrowser on Windows.

Changes or fixes:

• graf2d/win32gdk/gdk/src/gdk/win32/gdkim-win32.c: Replace mbstowcs with explicit UTF-8 decoding in gdk_nmbstowchar_ts, since ROOT's application manifest already declares UTF-8.

• graf2d/win32gdk/gdk/src/gdk/win32/gdkfont-win32.c: Add font fallback to the first available font in gdk_wchar_text_handle when no exact Unicode block match is found, with defensive null checks on the font list.

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR addresses #22450

[github-actions]

Test Results

    21 files      21 suites   3d 10h 49m 18s ⏱️
 3 864 tests  3 864 ✅ 0 💤 0 ❌
74 547 runs  74 547 ✅ 0 💤 0 ❌

Results for commit 073c555.

[wacfrr]

Hi, thanks for triggering the CI!

Regarding the clang-format failure on the CI, I apologize for the style discrepancy. I did try running clang-format locally before submitting, but it automatically triggered too many unintended changes . To avoid messing up the git history and keep this PR strictly focused on the essential encoding alignment, I decided to discard those global formatting changes and only modify the necessary parts.

Please let me know if you would like me to manually re-format just the newly added lines to match the project's style guide, or how we should best handle the formatting here. Thanks!

[bellenot]

@wacfrr no, don't worry about the formatting. And it looks good. I'll perform some more tests and then we'll see if it's good to go