An account whose source is gone was listed nowhere: `orphanedAccounts` had
no readers, and the credential behind it could not be reached at all —
`AppEnvironment.accountCredentialsManager(for:)` resolves the store from
the account's instance, and an orphan has none. The new accounts screen
lists them and removes them by account ID, asking every account-keyed
store because the backend is no longer knowable. The pre-revamp
instance-keyed item is deliberately left behind: identifying it needs the
instance too.
Deleting a source has always signed out and removed the accounts on it.
Both confirmations now say so, and name them.
The sign-in that files a credential against an account is now one
implementation instead of a private method on the source editor, so the
accounts screen and the new optional sign-in step after adding a source
attach a login exactly as the editor does. Extracted verbatim — the
identity-scope guard, the account re-resolve, the write-then-read-back
and the rollback are what keep a half-written login from looking real.
Signing out from the per-instance section now asks first, and both
sign-out paths run one cascade rather than two copies of it.
