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, 13 insertions, 2 deletions

diff --git a/src/notmuchworker.h b/src/notmuchworker.h
index 1d8c8c0..2dd766f 100644
--- a/src/notmuchworker.h
+++ b/src/notmuchworker.h
@@ -60,14 +60,25 @@ public:
 public slots:
     /// Runs a query. generation lets the UI discard results from a superseded
     /// query without the worker needing to know about cancellation.
+    /// `withRecipients` fills ThreadSummary::recipients by reading each
+    /// thread's To headers. OFF by default and deliberately opt-in: To is not
+    /// in notmuch's index, so this reads message FILES, at roughly 8.7 ms per
+    /// thread. Only a Sent query asks for it; turning it on for an inbox query
+    /// costs tens of seconds and changes nothing a user can see.
     void runQuery(const QString &query, quint64 generation,
-                  SortOrder sort = NewestFirst);
+                  SortOrder sort = NewestFirst, bool withRecipients = false);
 
     /// Loads the messages of one thread, oldest first. matchQuery is the
     /// user's current query; messages matching it render expanded, the rest
     /// as stubs.
+    /// `matchedOnly` drops the messages that did not match `matchQuery` rather
+    /// than rendering them as stubs. For the Sent view, where the thread is not
+    /// the unit the user is reading: a sent message pulls in the replies it
+    /// received, and a pane claiming to show what they sent then shows a
+    /// conversation. Ignored when `matchQuery` is empty, since nothing was
+    /// filtered and every message counts as matched.
     void loadThread(const QString &threadId, const QString &matchQuery,
-                    quint64 generation);
+                    quint64 generation, bool matchedOnly = false);
 
     /// Loads a thread as a reply TREE, for the message rows in the list.
     ///