In a cross-account view, marking a second account's email read/unread updated
the *active* account's folder counter instead of the email's. Two causes: the
optimistic counter update only touched `state.mailboxes` (the active account),
and JMAP mailbox ids can collide across accounts so the id match hit the wrong
folder.
Add applyMailboxCounterUpdate(): route the counter delta to the list that holds
the email's folders — the active account's `mailboxes` (incl. its shared
folders) for active-account/shared emails, otherwise that account's
`accountMailboxes[sourceClientAccountId]` entry. Use it in markAsRead.
Regression test: a 2nd-account email with a colliding inbox id decrements that
account's counter and leaves the active account's untouched.
Stefan Hildebrandt
befee332d2