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, 42 insertions, 0 deletions

diff --git a/tests/test_threadlistmodel.cpp b/tests/test_threadlistmodel.cpp
index e8a5fa8..97f7fde 100644
--- a/tests/test_threadlistmodel.cpp
+++ b/tests/test_threadlistmodel.cpp
@@ -38,6 +38,7 @@ private slots:
     void unreadStylingSurvivesAnAccountChip();
     void accountChipUsesTheConfiguredColour();
     void deletedThreadsAreRedAndStruckThrough();
+    void attachmentColumnIsFirstAndMarksOnlyTaggedThreads();
     void spamThreadsAreOrangeAndStruckThrough();
     void doomedStylingCoversEveryColumn();
     void ordinaryThreadsCarryNoRowColour();
@@ -472,5 +473,46 @@ void TestThreadListModel::modelPassesQtTester()
     model.clear();
 }
 
+void TestThreadListModel::attachmentColumnIsFirstAndMarksOnlyTaggedThreads()
+{
+    // Leftmost, and narrow: the point is to see an attachment without opening
+    // the thread, which only works if the column is never scrolled away.
+    QCOMPARE(ThreadListModel::AttachmentColumn, 0);
+
+    ThreadSummary plain = makeThread(QStringLiteral("t1"),
+                                     QStringLiteral("no attachment"));
+    ThreadSummary withFile = makeThread(QStringLiteral("t2"),
+                                        QStringLiteral("has one"));
+    // notmuch applies this tag itself while indexing, so no MIME parsing and
+    // no extra worker query are involved.
+    withFile.tags.append(QStringLiteral("attachment"));
+
+    ThreadListModel model;
+    model.appendBatch({ plain, withFile });
+
+    const QModelIndex plainCell =
+        model.index(0, ThreadListModel::AttachmentColumn);
+    const QModelIndex fileCell =
+        model.index(1, ThreadListModel::AttachmentColumn);
+
+    QVERIFY(model.data(plainCell, Qt::DisplayRole).toString().isEmpty());
+    QCOMPARE(model.data(fileCell, Qt::DisplayRole).toString(),
+             ThreadListModel::attachmentGlyph());
+
+    // The glyph must be something a font can draw. An unrenderable codepoint
+    // shows as a tofu box, which reads as breakage rather than as a marker.
+    QVERIFY(!ThreadListModel::attachmentGlyph().isEmpty());
+
+    // Only the marked thread gets a tooltip, or an empty cell would claim to
+    // have an attachment on hover.
+    QVERIFY(model.data(plainCell, Qt::ToolTipRole).toString().isEmpty());
+    QVERIFY(!model.data(fileCell, Qt::ToolTipRole).toString().isEmpty());
+
+    // The header carries no text: a label would set a minimum width far wider
+    // than the icon and defeat the narrow column.
+    QVERIFY(model.headerData(ThreadListModel::AttachmentColumn, Qt::Horizontal,
+                             Qt::DisplayRole).toString().isEmpty());
+}
+
 QTEST_MAIN(TestThreadListModel)
 #include "test_threadlistmodel.moc"