feat: orphan accounts

We now store whenever an account is considered an orphan, i.e. it couldn't be found during the latest sync with a provider (e.g. Woob).

This can happen if a sync returns accounts A, B, C on day 1, then accounts A, B on day 2. Account C would be an orphan.

It's discretely shown under the "last sync" field whenever that happens. We could add a pill in the account list in the future.