Fix warnings: remove unused import and variable

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

warzone/crates/warzone-client/src/storage.rs

@@ -1,7 +1,6 @@
 //! Local sled database: sessions, pre-keys, message history.
 
 use anyhow::{Context, Result};
-use base64::Engine as _;
 use warzone_protocol::ratchet::RatchetState;
 use warzone_protocol::types::Fingerprint;
 use x25519_dalek::StaticSecret;