v0.0.12: Encrypted backup/restore + history module

Protocol:
- history.rs: derive_history_key (HKDF from seed, info="warzone-history")
- encrypt_history / decrypt_history (ChaCha20-Poly1305, WZH1 magic)
- 2 new tests (roundtrip + wrong seed), total 19/19

CLI:
- `warzone backup [output.wzb]` — exports all sessions + pre-keys
  as encrypted blob (only your seed can decrypt)
- `warzone restore <input.wzb>` — imports backup, merges (no overwrite)
- Backup format: WZH1 magic + nonce + encrypted JSON

Storage:
- export_all() — dumps sessions + pre-keys as base64 JSON
- import_all() — merges backup data (skip existing entries)

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

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

@@ -1,6 +1,7 @@
 //! 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;
@@ -108,4 +109,73 @@ impl LocalDb {
             None => Ok(None),
         }
     }
+
+    /// Export all data as JSON (for encrypted backup).
+    pub fn export_all(&self) -> Result<serde_json::Value> {
+        let mut sessions = serde_json::Map::new();
+        for item in self.sessions.iter() {
+            if let Ok((k, v)) = item {
+                let key = String::from_utf8_lossy(&k).to_string();
+                sessions.insert(key, serde_json::json!(base64::Engine::encode(
+                    &base64::engine::general_purpose::STANDARD, &v
+                )));
+            }
+        }
+
+        let mut pre_keys = serde_json::Map::new();
+        for item in self.pre_keys.iter() {
+            if let Ok((k, v)) = item {
+                let key = String::from_utf8_lossy(&k).to_string();
+                pre_keys.insert(key, serde_json::json!(base64::Engine::encode(
+                    &base64::engine::general_purpose::STANDARD, &v
+                )));
+            }
+        }
+
+        Ok(serde_json::json!({
+            "version": 1,
+            "sessions": sessions,
+            "pre_keys": pre_keys,
+        }))
+    }
+
+    /// Import data from JSON backup (merges, doesn't overwrite existing).
+    pub fn import_all(&self, data: &serde_json::Value) -> Result<usize> {
+        let mut count = 0;
+
+        if let Some(sessions) = data.get("sessions").and_then(|v| v.as_object()) {
+            for (key, val) in sessions {
+                if let Some(b64) = val.as_str() {
+                    if let Ok(bytes) = base64::Engine::decode(
+                        &base64::engine::general_purpose::STANDARD, b64
+                    ) {
+                        // Only import if not already present
+                        if self.sessions.get(key.as_bytes())?.is_none() {
+                            self.sessions.insert(key.as_bytes(), bytes)?;
+                            count += 1;
+                        }
+                    }
+                }
+            }
+        }
+
+        if let Some(pre_keys) = data.get("pre_keys").and_then(|v| v.as_object()) {
+            for (key, val) in pre_keys {
+                if let Some(b64) = val.as_str() {
+                    if let Ok(bytes) = base64::Engine::decode(
+                        &base64::engine::general_purpose::STANDARD, b64
+                    ) {
+                        if self.pre_keys.get(key.as_bytes())?.is_none() {
+                            self.pre_keys.insert(key.as_bytes(), bytes)?;
+                            count += 1;
+                        }
+                    }
+                }
+            }
+        }
+
+        self.sessions.flush()?;
+        self.pre_keys.flush()?;
+        Ok(count)
+    }
 }