Marking mail as read left the sidebar's tag unread counts untouched — the
folder counts cleared, but a tag went on showing "47 unread" in bold until
the page was reloaded.
tagCounts is fetched from the server (Email/query per $label keyword) rather
than derived from state, and no read/unread mutation refreshed or adjusted
it. The per-mailbox unreadEmails counters were kept current by a local delta;
tags simply had no equivalent.
Add applyTagCountReadDelta alongside the existing mailbox-counter helpers and
apply it wherever the affected emails are known locally: markAsRead,
batchMarkAsRead, and setEmailKeywordsLocal. Only a genuine $seen flip moves a
count, so re-marking a read email as read cannot drift it, and unread is
clamped at zero. A tag's total is never touched by a read-state change.
markMailboxAsRead is the exception and refetches instead: it is a server-side
bulk operation over an entire mailbox, so it also marks emails that were never
loaded into state.emails, and a local delta would leave the counts high.
