* 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
# Pull Request Template
## Description
This PR includes the implementation of the new Compose Conversation form
in the conversation sidebar, replacing the old one.
## Type of change
- [x] New feature (non-breaking change which adds functionality)
## How Has This Been Tested?
### Loom video
https://www.loom.com/share/4312e20a63714eb892d7b5cd0dcda893?sid=9bd5254e-2b1f-462c-b2c1-a3048a111683
## Checklist:
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules
