fix: only bump file mtime when document transitions from modified to saved

[juliusknorr]

• Resolves: #
• Target version: main

Summary

Opening a file (e.g. a presentation) and closing it without making any edits was bumping the modified date in the Files listing to "seconds ago". A refresh reset it to the real mtime, since the file on disk was never touched.

Cause: the Doc_ModifiedStatus post-message handler updated the UI's mtime whenever args.Modified !== this.modified, which fires on both false → true (doc becomes dirty) and true → false (saved). For presentations, Collabora emits Modified: true shortly after open, triggering a spurious mtime bump.

Fix: only call FilesAppIntegration.updateFileInfo on the true → false transition, matching the original intent of the handler ("Update file metadata after save", f227260).

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Documentation (manuals or wiki) has been updated or is not required

[juliusknorr]

Actually this is not needed, Collabora seems to sent lots of messages on unchanged odp files