feat(crypto): stronghold persistence + passphrase device-key backup/restore

packages/shared/src/auth/device.ts

@@ -141,6 +141,17 @@ export async function forgetDevicePrivateKey(
   await secretStore.removeSecret(privateKeySecretName(userId, deviceId));
 }
 
+// Writes a device private key into the secret store. Used by the
+// backup-restore flow to re-import a key generated on another machine.
+export async function saveDevicePrivateKey(
+  secretStore: SecretStore,
+  userId: string,
+  deviceId: string,
+  privateKey: Uint8Array,
+): Promise<void> {
+  await secretStore.setSecret(privateKeySecretName(userId, deviceId), privateKey);
+}
+
 // Lightweight helpers for platforms that want to cache their current device id
 // in JSON storage (separate from the secret store, which only holds raw bytes).
 export const DEVICE_ID_STORAGE_KEY_PREFIX = 'chatapp.device_id';