libobs: Fix crash with non-ASCII characters in formatted filenames

[kellemar]

Description
Added os_strftime_utf8() to handle strftime correctly on Windows. The existing strftime() calls in os_generate_formatted_filename() now use this new function.

On Windows, strftime() returns strings in the system's ANSI codepage rather than UTF-8. The new function uses wcsftime() and converts to UTF-8 properly.

Motivation and Context
Fixes #12953

Using %Z (timezone name) in the filename format crashes OBS on German Windows because the timezone "Mitteleuropäische Zeit" contains an umlaut. The ANSI-encoded string gets passed to os_fopen() which expects UTF-8, causing MultiByteToWideChar to fail.

Same issue would happen with any locale that has non-ASCII characters in timezone names or other strftime output.

How Has This Been Tested?
I don't have a German Windows installation to test directly. The fix follows the same pattern OBS already uses elsewhere for Windows string handling (wcs functions + os_wcs_to_utf8).

Would appreciate if someone with a German Windows setup could verify.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format (https://github.com/obsproject/obs-studio/blob/master/.clang-format).
• I have read the contributing document (https://github.com/obsproject/obs-studio/blob/master/CONTRIBUTING.rst).
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[Dankirk]

I have looked into locales and character encodings a bit and have a pull request here #12624 that also fixes this. I can confirm the crash does happen on Windows with settings set to German and that this PR would also fix that.

The timezone name is a bit interesting though, since with our current OBS locale being minimal 'C' on Windows (and not the system locale), the %Z flag is still translated using the Windows display language. This only happens with %Z flag, for example %A (weekday name) properly follows locale settings ('C') and thus is always in English, even on German display language, locale and region.