Fix: update banner wiped by next sync; raise server-search timeout

Update banner: the "update available" notice shares the alert-banner
element, and the sync-success path calls hideAlert() unconditionally —
so the banner the auto-update poller raised was cleared within seconds
by the next sync and the user never saw it (the daemon log confirms the
poller has been detecting every release). The banner is now tagged
dataset.key="update-available"; hideAlert() leaves that key alone unless
forced, and the X dismiss button passes force:true.

Server search: a "Server"-checkbox search goes out to IMAP/Gmail and
can exceed the default 30s IPC ceiling, surfacing as "mailxapi timeout:
searchMessages". searchMessages now gets a 120s ceiling like the
attachment actions.
