CLI: add supported message pin/unpin operations with authorization checks

### Feature request

The Desktop app supports pinning messages, but the CLI does not expose equivalent operations.

Please add supported CLI operations to pin and unpin an existing message. They should enforce the same authorization and permission checks as other clients, return clear errors when the caller is not permitted, and make the resulting pin state observable through the CLI.

This would provide supported CLI/Desktop feature parity without requiring UI-only interaction.