fix: fix batch script Cyrillic paths and disk export backslash stripping on Windows

[Mr-Charvaka]

This PR resolves two Windows-specific issues:

1. Fix purge_nonexistent_images.bat with Cyrillic paths (purge_nonexistent_images.bat removes existing files #20915):

   • Sets the active code page to 65001 (UTF-8) during the file existence check loop.
   • Redirects sqlite3 output to a temp file and reads it using usebackq inside for /f, avoiding the cmd.exe character truncation/corruption parser bug when executing subprocesses under CP 65001.
   • Quotes path parameters in if not exist "%%b" to correctly handle folders/filenames containing spaces.

2. Fix Windows backslash path stripping in disk, gallery, and latex storage modules (Files are exported in a wrong location if using backslashes in the path #20981):

   • Cleans d->filename in the store() function on Windows by replacing all backslashes \ with forward slashes / before copying to the pattern string.
   • Forward slashes are natively supported as path separators by Windows filesystem APIs, and preventing backslashes in the pattern keeps them from being stripped as escape sequences during variable expansion.

[TurboGit]

@wpferguson : What's your view on those windows issues?

[wpferguson]

If this PR was generated with AI assistance, please note that.

What kind of testing was done? Was it only tested to make sure the problem was fixed or was testing also done to make sure there were no regressions or issues created by the fix?

[wpferguson]

What's your view on those windows issues?

@victoryforce would probably be the better one to ask about the Cyrillic issue.

I would recommend splitting this PR in 2 since it solves 2 problems that aren't really related except that they are both on Windows. It's easier to test one issue at and time and if necessary roll back one issue at a time.

[andriiryzhkov]

For the first part of this PR there is an earlier duplicate #20916

[Mr-Charvaka]

I’ll carefully review the changes, and if any problems surface, I’ll create a new PR after thorough verification and keep you informed. thanks @wpferguson @andriiryzhkov !

[andriiryzhkov]

@Mr-Charvaka : It would be much better to split this PR into two separate addressing each issue individually. Especially considering first one is duplicate fix. Not saying it is better or worse though.

[andriiryzhkov]

@Mr-Charvaka : The purge_nonexistent_images.bat fix substantially overlaps with the open PR #20916 (same #20915 bug, same workaround pattern). That PR also catches two things this one doesn't:

• %ids% → !ids! in the two DELETE statements – without it the actual purge silently fails for multiple images
• An endlocal for cleanup

Your codepage save/restore is nicer than #20916's blanket chcp though.

Could you drop the .bat changes here and let #20916 land? Your save/restore could go on top as a one-liner. Keeps the #20981 disk/gallery/latex backslash fix cleanly reviewable on its own.