feat(sent): add a Sent view, flat and by recipient

Adds a `sent` key to [account.*] naming that account's sent folder, and a
Sent button beside the saved queries that composes its query from every
account carrying one. An account without the key is omitted silently, as a
real account may keep no sent mail locally. With no account selected the
button spans all of them; selecting one narrows it through the existing
scope wrap rather than a second path.

Composed at run time rather than shipped as a [queries] entry. A saved query
is one fixed string: it cannot narrow to the selected account, and it goes
stale the moment an account is added or a provider renames a folder.

The design and the measurements behind it are in
docs/superpowers/specs/2026-08-11-sent-mail-design.md. Three things there are
worth repeating here.

The composed path is QUOTED, and that is load-bearing. A real provider nests
its sent folder under a bracketed parent, and "[" and "]" are Xapian syntax:
unquoted, the query parses rather than matches and returns nothing while
looking entirely plausible. Composition happens in one place so there is one
chance to get it right, and a bracketed path is pinned in a test.

Recipients are opt-in per query, which is a performance contract rather than
a preference. notmuch_message_get_header(m, "To") is not served from the
index, it reads the message file: folding every thread of a 4411-thread
inbox took 38.2 seconds against 251 ms for the 601-thread sent view. The
worker skips the walk entirely unless asked, and the refresh path carries the
same flag so a background sync cannot blank the column mid-read. Always
folding is mutation-tested: the data would be right and only the cost wrong,
which nothing else here would notice.

The messages reached through the thread are owned by it and freed with it, so
recipientsOf() holds them raw and finishes while the thread is alive, exactly
as walkReplies does. An NmMessage wrapper there is a double-free.

Sent mail is presented flat, and the pane follows. A message you sent
otherwise drags in the replies you received, so a view labelled Sent shows
conversations rather than what you sent. ThreadListModel::setFlatMode() makes
hasChildren() and ReplyCountRole answer differently and changes nothing else;
runQuery() sets it on EVERY run, so any other query restores the tree on its
way through and the flag cannot outlive the button that set it. The pane
needed its own fix for the same reason: the single-message path depends on a
field only filled when a thread is expanded, which never happens in a flat
list, so loadThread() gained matchedOnly and drops the messages that did not
match instead of rendering them as stubs.

Recipients replace the sender through the existing SendersRole rather than a
new one, so the delegate needs no branch and cannot disagree with the model
about which name a row shows. It falls back to the sender when a To header is
absent or unparseable, since a blank where a name belongs reads as a
rendering fault.

Address parsing uses GMime: a display name may contain a comma, so
"Rossi, Mario" <m@example.org>, info@example.net is two addresses and
splitting reports three. internet_address_list_parse returns NULL for an
empty string, which is a crash if unguarded.

Backlog item 63.

1 files changed, 104 insertions, 0 deletions

diff --git a/tests/test_threadlistmodel.cpp b/tests/test_threadlistmodel.cpp
index 947a6b3..1fb8a1f 100644
--- a/tests/test_threadlistmodel.cpp
+++ b/tests/test_threadlistmodel.cpp
@@ -81,6 +81,9 @@ private slots:
     void reconcileWithAnIdenticalResultChangesNothing();
     void reconcileMovesAThreadBumpedByANewReply();
     void reconcileKeepsAMovedRowsPersistentIndex();
+    void flatModeOffersNoExpanderAndNoReplyCount();
+    void flatModeIsOffByDefaultAndReversible();
+    void recipientsReplaceTheSenderWhenPresent();
 };
 
 static ThreadSummary makeThread(const QString &id, const QString &subject)
@@ -1420,5 +1423,106 @@ void TestThreadListModel::reconcileKeepsAMovedRowsPersistentIndex()
     QCOMPARE(moved.row(), 2);
 }
 
+void TestThreadListModel::flatModeOffersNoExpanderAndNoReplyCount()
+{
+    // A sent message lives on its own: the user's mental model of "what I sent"
+    // is a list, not a set of conversations, and a thread pulled in whole shows
+    // the replies they received under a view that claims to be their outbox.
+    //
+    // Deliberately not a second model or a filtered query. The expander is
+    // driven by hasChildren() and the card's count by ReplyCountRole, both
+    // already here, so flat mode is those two answering differently.
+    ThreadListModel model;
+    model.appendBatch({ makeThread(QStringLiteral("t1"),
+                                   QStringLiteral("Subject")) });
+
+    const QModelIndex thread = model.index(0, 0);
+    QVERIFY(thread.isValid());
+
+    // The tree shape, before anything is turned off. Guards the assertions
+    // below: a test whose subject was already flat would pass either way.
+    QVERIFY2(model.hasChildren(thread),
+             "the fixture thread is not expandable, so this proves nothing");
+    QCOMPARE(model.data(thread, ThreadListModel::ReplyCountRole).toInt(), 1);
+
+    model.setFlatMode(true);
+
+    QVERIFY2(!model.hasChildren(thread),
+             "a flat list still offered an expander");
+    QCOMPARE(model.data(thread, ThreadListModel::ReplyCountRole).toInt(), 0);
+
+    // rowCount has to agree, or the view draws an expander it cannot open, or
+    // opens onto rows the card said were not there.
+    QCOMPARE(model.rowCount(thread), 0);
+
+    // The thread itself is still a row. Flat means one row per thread, not
+    // fewer threads.
+    QCOMPARE(model.rowCount(), 1);
+}
+
+void TestThreadListModel::flatModeIsOffByDefaultAndReversible()
+{
+    // The whole condition the user set for this feature: it must not leak into
+    // any other view. Off by default is what guarantees that, and returning to
+    // false has to restore the tree rather than leaving the model flattened
+    // for the next query.
+    ThreadListModel model;
+    model.appendBatch({ makeThread(QStringLiteral("t1"),
+                                   QStringLiteral("Subject")) });
+
+    const QModelIndex thread = model.index(0, 0);
+    QVERIFY2(model.hasChildren(thread),
+             "a fresh model is flat, so every ordinary view lost its replies");
+
+    model.setFlatMode(true);
+    QVERIFY(!model.hasChildren(thread));
+
+    model.setFlatMode(false);
+    QVERIFY2(model.hasChildren(thread),
+             "leaving flat mode did not restore the tree");
+    QCOMPARE(model.data(thread, ThreadListModel::ReplyCountRole).toInt(), 1);
+}
+
+void TestThreadListModel::recipientsReplaceTheSenderWhenPresent()
+{
+    // In a Sent view the sender is the user on every row, so the card shows
+    // who it went TO instead. One role, so the delegate needs no branch and
+    // cannot disagree with the model about which name a row is showing.
+    ThreadListModel model;
+
+    ThreadSummary sent = makeThread(QStringLiteral("t1"),
+                                    QStringLiteral("Preventivo"));
+    sent.authors = QStringLiteral("You");
+    sent.recipients = QStringLiteral("Mario Rossi");
+
+    // No recipients: an ordinary view, where authors is the answer. Same
+    // fixture otherwise, so the difference is the field and nothing else.
+    ThreadSummary received = makeThread(QStringLiteral("t2"),
+                                        QStringLiteral("Newsletter"));
+    received.authors = QStringLiteral("Carol");
+
+    model.appendBatch({ sent, received });
+
+    QCOMPARE(model.data(model.index(0, 0),
+                        ThreadListModel::SendersRole).toString(),
+             QStringLiteral("Mario Rossi"));
+    QCOMPARE(model.data(model.index(1, 0),
+                        ThreadListModel::SendersRole).toString(),
+             QStringLiteral("Carol"));
+
+    // A thread whose To could not be parsed falls back rather than showing an
+    // empty name. The fold returns an empty string for a malformed or absent
+    // header, and a blank where a sender belongs reads as a rendering fault.
+    ThreadSummary unparseable = makeThread(QStringLiteral("t3"),
+                                           QStringLiteral("Broken"));
+    unparseable.authors = QStringLiteral("You");
+    unparseable.recipients = QString();
+    model.appendBatch({ unparseable });
+
+    QCOMPARE(model.data(model.index(2, 0),
+                        ThreadListModel::SendersRole).toString(),
+             QStringLiteral("You"));
+}
+
 QTEST_MAIN(TestThreadListModel)
 #include "test_threadlistmodel.moc"