An inactive account keeps its credential and syncs like a rename, but
drops out of every picker and cannot be used until activated again.
Deactivating the selected account switches to the local context as a
published choice, so other devices follow instead of falling back.
MAC1: switcher rows are Toggles on macOS so the checkmark really renders,
and the settings row names its context via LabeledContent with a tooltip.
MAC4: accessible names are words, never raw SF Symbol identifiers.
MAC14: new displayStatus(for:) surfaces instanceDisabled on account rows,
badges, and the switcher tooltip without disturbing status(for:) consumers.
MAC16: disambiguated labels guard the username branch and fall back to the
source name only when rows would still collide.
Also carries the new Localizable strings for the whole macOS fix batch.
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.
