refactor(search): carry SearchMode instead of bool extend

Four signatures, no behaviour change: the two shipped operations map to
Replace and Narrow. runSearchFromPane becomes a switch and gains the
Exclude arm, which nothing can reach until the menu entry exists.

Seven call sites across three test files moved with it, two more than
the plan predicted: test_messageview and test_mainwindow also drive
these signals directly.

mainwindow.h and messagedetailsdialog.h now include searchterm.h for
the type; messageview.h already did.

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

1 files changed, 5 insertions, 4 deletions

diff --git a/src/messageview.h b/src/messageview.h
index 3e600f0..877064e 100644
--- a/src/messageview.h
+++ b/src/messageview.h
@@ -182,15 +182,16 @@ signals:
 
     /// The user chose a search from one of the pane's context menus.
     ///
-    /// `extend` narrows the current query rather than replacing it. The view
-    /// does not know what the query bar holds and must not: the window owns
-    /// that field and does the combining.
+    /// `mode` says whether to replace the query bar, narrow it, or narrow it
+    /// by everything that is not this value. The view does not know what the
+    /// query bar holds and must not: the window owns that field and does the
+    /// combining.
     ///
     /// Separate from queryRequested(), which carries a gate against a link in
     /// a rendered document driving the thread list. These menus are chrome
     /// built by our own code from values we extracted, so they need no gate,
     /// and widening the existing signal would change what that gate protects.
-    void searchRequested(const QString &query, bool extend);
+    void searchRequested(const QString &query, SearchTerm::SearchMode mode);
 
 protected:
     /// Turns Ctrl+wheel over the body into zoom, and Ctrl+middle-click into a