Function sortGalleryFiles

Sort page attachments by a gallery's order attribute.

Returns a new array (the shared PageFileData list from the page context is never mutated). Rules:

• name / name desc: code-point comparison of the filename; desc flips the comparison. Names are unique per page, so no tie-break.
• created_at / created_at desc: files with createdAt come first, compared numerically (desc flips only this comparison); equal timestamps tie-break on name ascending (also under desc). Files without createdAt always sort last, keeping their given order.