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>

1 files changed, 15 insertions, 0 deletions

diff --git a/src/searchterm.cpp b/src/searchterm.cpp
index acc64c6..6bfb521 100644
--- a/src/searchterm.cpp
+++ b/src/searchterm.cpp
@@ -90,4 +90,19 @@ QString extend(const QString &existing, const QString &addition)
     return QStringLiteral("(%1) AND (%2)").arg(left, right);
 }
 
+QString exclude(const QString &existing, const QString &addition)
+{
+    const QString left = existing.trimmed();
+    const QString right = addition.trimmed();
+
+    if (right.isEmpty())
+        return left;
+    // NOT a replace, unlike extend(): see the header. An empty left would make
+    // this "everything except", which no right-click asked for.
+    if (left.isEmpty())
+        return QString();
+
+    return QStringLiteral("(%1) AND NOT (%2)").arg(left, right);
+}
+
 }  // namespace SearchTerm