Add descriptor coordination to keep-mobile UniFFI bindings

Expose wallet descriptor operations through UniFFI for keep-android:

- `wallet_descriptor_list()` → `Vec<WalletDescriptorInfo>`
- `wallet_descriptor_export(group_pubkey, format)` → `String`
- `wallet_descriptor_delete(group_pubkey)`
- Potentially: initiate/respond to descriptor coordination sessions

Storage and CLI commands exist — need the UniFFI bridge.