#60 Add keyring-based group sharing for multi-user access control

Subtasks

• closed #55 Add upload/download support for keyring encryption
• closed #56 Add keyring member management (add-member, remove-member)
• closed #57 Add CLI keyring commands and local group key store
• closed #58 Add keyrings core module with create and list operations
• closed #59 Implement symmetric key wrapping primitives in crypto.rs