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, 18 insertions, 1 deletions

diff --git a/src/mainwindow.cpp b/src/mainwindow.cpp
index 2a79988..1385f01 100644
--- a/src/mainwindow.cpp
+++ b/src/mainwindow.cpp
@@ -100,9 +100,18 @@ void MainWindow::restoreUiState()
         m_splitter->restoreState(splitter);
     }
 
+    // A header blob saved against a different set of columns must be
+    // discarded, not restored. QHeaderView::restoreState() returns TRUE for a
+    // blob with fewer sections than the model and applies the old widths to
+    // the wrong columns: adding the attachment column in front shifted every
+    // saved width one place right, silently mangling the layout with no error
+    // to detect it by (verified on Qt 6.11). The column count is stored
+    // alongside and the blob is only used when it still matches.
     const QByteArray header = state.value(QStringLiteral("threadlist/header"))
                                   .toByteArray();
-    if (!header.isEmpty()) {
+    const int savedColumns =
+        state.value(QStringLiteral("threadlist/columns")).toInt();
+    if (!header.isEmpty() && savedColumns == ThreadListModel::ColumnCount) {
         m_threadView->horizontalHeader()->restoreState(header);
     }
 
@@ -123,6 +132,9 @@ void MainWindow::saveUiState() const
     state.setValue(QStringLiteral("window/splitter"), m_splitter->saveState());
     state.setValue(QStringLiteral("threadlist/header"),
                    m_threadView->horizontalHeader()->saveState());
+    // Guards the blob above: see restoreUiState().
+    state.setValue(QStringLiteral("threadlist/columns"),
+                   int(ThreadListModel::ColumnCount));
     state.setValue(QStringLiteral("message/zoom"), m_messageView->zoomFactor());
 }
 
@@ -275,6 +287,11 @@ void MainWindow::buildUi()
 
     // Starting widths only; a drag overrides them, and they are what the
     // saved-widths item will persist.
+    // Without this the attachment column cannot be narrow at all: the default
+    // minimum section size is 58px on this platform, and setColumnWidth()
+    // clamps to it silently rather than reporting the smaller value back.
+    m_threadView->horizontalHeader()->setMinimumSectionSize(24);
+    m_threadView->setColumnWidth(ThreadListModel::AttachmentColumn, 28);
     m_threadView->setColumnWidth(ThreadListModel::DateColumn, 130);
     m_threadView->setColumnWidth(ThreadListModel::AuthorsColumn, 180);
     m_threadView->setColumnWidth(ThreadListModel::SubjectColumn, 520);