Add detailed specification for file and pack sharing system

Commit notes

- Document existing sharing infrastructure (shareLinks, sharePermissions) - Specify upload links feature for receiving files from collaborators - Design comments and annotations system for feedback on shared content - Plan activity tracking and notification system - Outline frontend routes and components needed - Include database migration summary and API routes - Prioritize implementation phases (MVP -> Advanced)

Co-authored-by: armin.naimi <[email redacted]>