Lock vault on a new local account with no vault file opens onboarding instead of the unlock screen #592

A local account that has no secrets.enc yet (created, nothing saved) lands on the first-launch onboarding screen after Lock vault, instead of the locked screen.

Steps

1. Create a local account with a master password. Don't add anything that writes a secret (no saved passwords or keys), so secrets.enc is never created.
2. Settings → Account → Lock now with Lock vault.
3. Voltius reloads.

Expected: the locked screen asks for the master password.
Actual: the first-launch screen ("Choose how you want to use Voltius"). Picking an option there creates a new account over the old one's keychain entries.

Why

SplashScreen decides locked vs first launch with hasAccountToUnlock() = getVaultStatus().exists || isServerMode(). A local account with no vault file matches neither. #581 fixed the same case for cloud accounts (server mode); local accounts still fall through. The account's account_id/mode keychain entries are still there, so the splash could treat "an account exists" as locked.

Found while building the screenshot harness for #591 (spec 2 of #510). Not caused by it.