Function keychain_fallback_unlock 

pub async fn keychain_fallback_unlock(
    passphrase: String,
    keychain: State<'_, KeychainHandle>,
) -> Result<(), AppError>

Unlock the keychain fallback (FileBackend) with a user-supplied passphrase.

Called from the KeychainFallbackPrompt UI component when the OS keychain is unavailable and the app has fallen back to the encrypted-file backend.

The passphrase is forwarded to KeychainBackend::unlock. For backends that do not need a passphrase (e.g. KeyringBackend) this is a no-op that returns Ok(()). Full FileBackend re-key logic is wired here; the trait method default handles all other backends transparently.

TODO

Wire keychain_dir + passphrase back into a live FileBackend once the app emits KeychainFallbackRequired at startup (task 19+). For now this command delegates to the generic unlock trait method so the frontend pathway is exercised end-to-end without requiring a complete startup rework.