feat(sync): show unsynced edits and offer to sync on exit

Tagging changes the notmuch index at once, but the mail store only hears
about it on the next sync, and nothing said so. Quitting with tagging
outstanding was silent. Items 18 and 19 of the usability backlog, built
together because the second needs the first's counter.

The counter cannot be QUndoStack::isClean(), which is the obvious
candidate and the wrong one: the undo stack is cleared on every query,
since its entries refer to rows the new result set discards. Tag a
thread, run any query, and the stack is empty while the change is still
unsynced. m_pendingEdits is its own count, incremented where a write is
CONFIRMED rather than where one is sent, so an optimistic update the
worker later rejects cannot leave the indicator claiming an edit that
never landed. Only a successful sync resets it: clearing on failure would
assert the changes had reached the mail store when the sync is exactly
what failed to put them there.

It is shown in the status bar, hidden entirely at zero, and described as
a lower bound rather than a guarantee, since an external notmuch run can
carry changes over without this application noticing.

On exit, sync_on_exit in [general] takes ask, always or never. Three
values rather than a bool because "prompt me", "just do it" and "do
nothing" are three behaviours and true/false expresses two; an unknown
value warns by name, since a typo there silently changes what happens to
unsynced work. The prompt offers three buttons for the same reason: a
user who hit Quit by mistake needs a way back that is not "sync". A sync
started at exit holds the window open until it finishes rather than being
killed mid-run, and a sync that FAILS does not quit, because quitting
there would discard the user's choice silently. With no sync command
configured the prompt degrades to a plain warning instead of offering a
sync that cannot run.

This is not a destructive-action confirmation of the kind CLAUDE.md
forbids. Those cover tag mutations, which keep undo instead of a dialog.
This asks about losing work at the one point where undo cannot help.

The tagsApplied lambda became a named slot, which is better structure and
also what lets a test drive it: the worker is deliberately parentless
because it moves to its own thread, so reaching it with findChild to emit
the real signal cannot work, and contorting the test to try was the wrong
instinct. Testing a modal needed its own care. A test that sends a close
event hangs forever if an unexpected dialog opens, because the modal
spins its own event loop; CloseProbe polls for activeModalWidget, closes
it and records that one appeared, turning "a dialog opened" into an
assertion rather than a hang.

Also removes a stray qDebug left in the open_thread action by the earlier
Enter-key investigation, which had reached two commits.

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

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