feat(threads): mark an opened thread read after a delay

Opening a thread left it tagged unread, so the unread count never
matched what had actually been read and the app was quietly wrong every
day it was used. Item 6 of the usability backlog.

A single-shot timer, armed when a thread is selected and restarted rather
than stacked, so arrowing down a list marks only the thread still
selected when it fires and not every one passed through. Configurable
through mark_read_delay_ms in [general], defaulting to 2000: zero marks
read at once, and any negative value disables the behaviour, which is why
the value is neither clamped nor warned about at either end.

The automatic change deliberately does NOT go on the undo stack. It
routes through sendThreadTagChange() rather than tagSelected(), because
undoing an action the user never took is worse than leaving a thread
read, and toggle_unread already gives them a direct way back. It still
funnels through the single applyTags path; what differs is only whether
the inverse is pushed, which is a window-level decision above the worker.
An explicit toggle_unread cancels any pending timer, or marking a thread
unread by hand would be reversed a moment later and the key would look
broken.

Two guards beyond the plan, both from asking what happens when a timer
outlives the thread it was armed for. Arming is skipped for a thread that
is not unread, so no write is scheduled that would change nothing, and
the handler re-checks that its thread is still selected and still unread
before writing, so a stale timer does nothing rather than tagging the
wrong thread.

The plan expected the rapid-arrow case to need a database and a manual
check. It needs neither: ThreadListModel takes threads through
appendBatch(), so the case is unit-tested. All three tests were confirmed
to fail against deliberately broken versions, one arming for read threads
and one creating a timer per selection instead of restarting one.

Item 7 is closed in the same pass. The user verified against real mail
that HTML messages already open as HTML, which is what the item asked
for, so it is recorded as done with no code changed. The prefer_html key
it floated was not added: nobody has asked to default to plain text, and
Ctrl+H already switches a thread by hand.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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