Fork Approval Signature

- Added when a user **forks** a page and explicitly signs it. - Indicates they have reviewed and accept the forked version. - Can serve as a signal to others that the page is safe to use or remix. Fields: - `sig_type`: `"fork-approval"` - `forked_by`: identity of the forker - `original_page_hash`: hash of source page - `signature`: full fork signature