fix(sync): hold tag edits made during a background sync

A tag edit sent while another process holds notmuch's write lock does not
fail: the read-write open blocks and then succeeds. Measured against
Slackware's notmuch, 9.158s against a 12s hold, status SUCCESS. Since the
worker is a single thread, that blocked open holds up every read queued
behind it, so the message pane freezes on whichever thread was selected
first and replays the queue when the lock releases.

The window now defers instead. While SyncMonitor reports a sync running, a
tag change is held rather than sent, and flushed when the sync ends. The
optimistic update stands in the meantime, so the row keeps its tag and the
edit still counts toward the unsynced indicator, which is what the quit
prompt reads.

The original diagnosis was that the open fails and the edit is discarded,
and a retry was built on it. That was wrong: the error branch in
notmuchworker.cpp is unreachable through lock contention. The premise was
taken from a plausible-looking error path without provoking the condition,
and measurement disproved it. The backlog entry records this rather than
quietly correcting it.

Verified by hand against a real blocking open, which the tests cannot reach:
they drive the deferral through the meta-object and never take a lock. Both
locks held for 100s with a tag edit made during the hold. Row kept the tag,
status did not expire, indicator rose, window stayed responsive, held edit
sent itself on release.

The 2s SyncMonitor polling window is knowingly left open: a sync starting
between polls is invisible for up to 2s and an edit there still blocks.
SyncMonitor::lockHeldIn() would close it at the cost of one file read per
tag action, and is recorded as the option to revisit.

Also fixes revertPendingTagChange() clearing the entire undo stack after any
rejected write, found while working on this.

Backlog: item 37 done, and item 46 added for a test that fails only under
the offscreen platform, where an 800x800 screen clamps a restored 940px
window. Pre-existing and unrelated; the suite is green otherwise.

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