Implement permission utilities (task 9)

- 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

.kiro/specs/user-auth-and-sharing/tasks.md

@@ -119,7 +119,7 @@
   - **Property 16: Account deletion cascades to lists**
   - **Validates: Requirements 4.5**
 
-- [ ] 9. Implement permission utilities
+- [x] 9. Implement permission utilities
   - Create canAccessList function
   - Create canEditList function
   - Create canDeleteList function