Administrators have access to all inboxes in an account but the
validate_inbox_access filter only checked InboxMember records, returning
401 for admins not explicitly added as inbox members.
* feat: add per-inbox signature management
- Introduced `InboxSignature` model to manage signatures specific to each inbox.
- Added API endpoints for fetching, creating, updating, and deleting inbox signatures.
- Updated UI components to support inbox-specific signatures, including overrides for signature position and separator.
- Implemented a new composable `useInboxSignatures` for managing inbox signatures in the frontend.
- Enhanced existing components to utilize inbox signatures, including the reply box and message signature settings.
- Added tests for the new inbox signatures functionality, ensuring proper behavior of the API and model validations.
- Updated translations for new UI elements related to inbox signatures.
* feat: implement inbox access validation and add related tests
* feat: enhance inbox signatures fetching and management logic
