Function saveConfig

apps/desktop/src/main/vault.ts:594–638

(cfg: PersistedConfig)

 *  to `<path>.bak` so a crash mid-rename can't strand the user without a
 *  recoverable config. */
export async function saveConfig(cfg: PersistedConfig): Promise<void> {
  const sanitized = sanitizeForPersist(cfg)
  const target = configPath()
  const tmp = `${target}.${process.pid}.${Date.now()}.tmp`
  const backup = configBackupPath()
  const payload = JSON.stringify(sanitized, null, 2)
  await fs.mkdir(path.dirname(target), { recursive: true })
  await cleanupStaleConfigTmpFiles()

  // Write + fsync the temp file so the bytes are on disk before we rename.
  const handle = await fs.open(tmp, 'w')
  try {
    await handle.writeFile(payload, 'utf8')
    try {
      await handle.sync()
    } catch (syncErr) {
      // fsync isn't supported on every filesystem. Don't abort the save.
      console.warn('fsync failed for config temp file', syncErr)
    }
  } finally {
    await handle.close()
  }

  // Keep the previous good file as a backup before overwriting. Best-effort:
  // missing primary just means there's nothing to back up yet.
  try {
    await fs.copyFile(target, backup)
  } catch (err) {
    if (!isMissingFileError(err)) {
      console.warn('Failed to refresh config backup', err)
    }
  }

  try {
    await fs.rename(tmp, target)
  } catch (err) {
    // Rename failed — clean up the temp file so it doesn't accumulate.
    try {
      await fs.unlink(tmp)
    } catch {
      /* ignore */
    }
    throw err
  }
}

/**
 * Atomically write a file: temp file + fsync + rename. The rename is atomic, so