feat: show a paperclip for threads with attachments

An attachment was only discoverable by opening the thread. A narrow
leftmost column now marks the threads that carry one.

No new worker query is involved: notmuch applies the "attachment" tag
while indexing, so ThreadSummary already holds what this needs. The
marker is a glyph rather than an icon resource, which ships no new asset
and inherits the row font, so it strikes through with a doomed thread
like every other cell. It falls back to "*" where the system font cannot
draw U+1F4CE, since an unrenderable codepoint reads as breakage rather
than as a marker.

Two silent Qt behaviours had to be handled, both found by probe:

QHeaderView::restoreState() returns true for a blob saved against fewer
columns and applies the old widths shifted one place right. Adding a
column in front would therefore have mangled every existing saved
layout with no error to detect it by. The column count is now stored
beside the blob and a mismatch discards it, so the widths reset once on
upgrade instead of landing on the wrong columns.

QHeaderView's default minimumSectionSize is 58px on this platform, and
setColumnWidth() clamps to it without reporting the smaller value back,
so the column could not be narrow at all until it was lowered.

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

1 files changed, 44 insertions, 0 deletions

diff --git a/tests/test_mainwindow.cpp b/tests/test_mainwindow.cpp
index b4d9e4a..ab904f9 100644
--- a/tests/test_mainwindow.cpp
+++ b/tests/test_mainwindow.cpp
@@ -25,10 +25,13 @@
 #include <QStandardPaths>
 #include <QTemporaryDir>
 
+#include <QTableView>
+
 #include "config.h"
 #include "keymap.h"
 #include "mainwindow.h"
 #include "messageview.h"
+#include "threadlistmodel.h"
 
 /// MainWindow is mostly wiring, and the parts that need a real database are
 /// still verified manually. What is checked here is the action registry: the
@@ -47,6 +50,7 @@ private slots:
     void uiStateIsNotWrittenIntoTheUserConfig();
     void uiStateSurvivesARestart();
     void missingUiStateLeavesTheDefaults();
+    void headerStateFromADifferentColumnLayoutIsDiscarded();
 };
 
 void TestMainWindow::everyKnownActionIsRegistered()
@@ -222,6 +226,46 @@ void TestMainWindow::missingUiStateLeavesTheDefaults()
     QStandardPaths::setTestModeEnabled(false);
 }
 
+void TestMainWindow::headerStateFromADifferentColumnLayoutIsDiscarded()
+{
+    // The upgrade hazard: a 0.3.0 state file holds a three-column header blob,
+    // and 0.4.0 added the attachment column in front. QHeaderView::
+    // restoreState() returns TRUE for a blob with fewer sections than the
+    // model and applies the old widths shifted one column right, mangling the
+    // layout with no error to detect it by (verified on Qt 6.11). The stored
+    // column count is what makes that detectable.
+    QStandardPaths::setTestModeEnabled(true);
+    QFile::remove(MainWindow::uiStatePath());
+
+    {
+        const Config config;
+        MainWindow window(config);
+        window.close();
+    }
+
+    // Forge a state file from an older layout: same blob, wrong column count.
+    {
+        QSettings state(MainWindow::uiStatePath(), QSettings::IniFormat);
+        state.setValue(QStringLiteral("threadlist/columns"),
+                       int(ThreadListModel::ColumnCount) - 1);
+        state.setValue(QStringLiteral("threadlist/header"),
+                       QByteArray("not a header this model could have saved"));
+    }
+
+    // Constructing must not apply it, and must not crash on the garbage blob.
+    const Config config;
+    MainWindow reopened(config);
+
+    auto *view = reopened.findChild<QTableView *>();
+    QVERIFY(view);
+    QCOMPARE(view->columnWidth(ThreadListModel::AttachmentColumn), 28);
+    QCOMPARE(view->columnWidth(ThreadListModel::DateColumn), 130);
+    QCOMPARE(view->columnWidth(ThreadListModel::SubjectColumn), 520);
+
+    QFile::remove(MainWindow::uiStatePath());
+    QStandardPaths::setTestModeEnabled(false);
+}
+
 // Constructing a MainWindow needs a QApplication and a platform plugin. The
 // test has no display under ctest, so it runs offscreen unless the caller
 // asked for something else.