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.
Switching accounts was buried in Subscriptions settings behind a picker
and a confirmation dialog, and it wrote two stores that could disagree.
One shared switcher now sits in the Subscriptions and Playlists
toolbars, the macOS sidebar footer and the Settings root, and every path
goes through AppEnvironment.switchDataContext(to:) so a switch behaves
the same wherever it is made. Switching is one tap: it is reversible, so
there is nothing to confirm.
Rows come from DataContextValidator.availableContexts, name themselves
through displayName(for:), and carry a status suffix when the account is
not signed in — plus the instance host when two accounts share a name.
tvOS gets the same rows as a full-screen picker.
Subscriptions also explains itself when the selected account cannot
answer: a thin banner names the cause and says the data below is local.
This is the first consumer of the validator's unavailability, and
correctIfContextRemoved() now runs on foreground activation, where
SubscriptionAccountValidator used to auto-correct any selection it could
not immediately verify.
With the switcher reading the context directly, the legacy
subscriptionAccount setting has no readers left: the setting, its
validator, the mirror that kept it in step and the feed cache's
instance-keyed fallbacks all go. The cache invalidation the mirror also
carried stays, under its own name. AccountsManager still reads the raw
legacy key for the one-way context migration.
