- Created lib/permissions.ts with four utility functions:
- canAccessList: Check if user can view a list
- canEditList: Check if user can edit a list or its items
- canDeleteList: Check if user can delete a list (owner only)
- getPermission: Get user's permission level for a list
- Added comprehensive unit tests in lib/__tests__/permissions.test.ts
- All 24 tests passing
- Validates Requirements 4.3, 6.2, 6.3
