feat(search): add SearchTerm::exclude

Parenthesises both sides, as extend() does: unparenthesised, a
disjunction in the query bar binds so the exclusion covers only its last
term and leaves the excluded mail on screen, with nothing reporting an
error.

An empty existing query returns empty rather than the addition alone,
which is where this deliberately differs from extend(). Excluding from
nothing means the whole Maildir minus one value; the menus will grey the
entry out and this is the second layer.

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

0 files changed, 0 insertions, 0 deletions