Tags applied to a shared/group-mailbox message did not persist. Custom
keywords (:*), and were written via Email/set
against the reaching client's primary account instead of the email's
owning account, so the server returned notUpdated without an error and
the change was lost on the next reload.
toggleStar already threaded an accountId through (#281); the keyword
methods did not. Add an optional accountId to updateEmailKeywords and
setKeyword and resolve it at the call sites from the email's source
account (sourceClientAccountId / sourceAccountId), matching the existing
delete/archive routing. Personal sources resolve to the account itself,
so behavior there is unchanged.
