feat: add letters feature with detail view and listingHEADmain

- Introduced new routes for letters and their details.
- Created pages for displaying letter details and listing letters.
- Added new types for letters, including participants, messages, and attachments.
- Implemented API calls for fetching letters and managing messages (reply, edit, delete).
- Enhanced stats to include unread letters and pending districts for staff users.
- Updated styles for letters and their components.
- Added privacy settings for letters (public and friends).
- Modified user model to include letter privacy settings.
- Improved error handling and user feedback in the UI.

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