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, 31 insertions, 0 deletions

diff --git a/src/config.h b/src/config.h
index bcbfb34..e3c5b6e 100644
--- a/src/config.h
+++ b/src/config.h
@@ -36,6 +36,18 @@ struct Account
     QString maildir;  ///< Relative to notmuch's database.path.
     QString drafts;   ///< Unused in v1; send is v2.
 
+    /// The account's sent folder, relative to maildir. Optional and empty for
+    /// an account that has none, which is a real case rather than a
+    /// misconfiguration: an account may keep no sent mail locally at all.
+    ///
+    /// A key rather than a <maildir>/Sent convention because the folder is not
+    /// uniform across providers. Measured across one real setup: two accounts
+    /// use `Sent`, two nest a localised name under a bracketed parent
+    /// (`[Provider]/Posta inviata`), and one has no sent folder whatsoever. A
+    /// convention would produce an empty view for the nested ones and a wrong
+    /// one for the account that has none.
+    QString sent;
+
     /// Chip colour in the thread list. Invalid when unset, in which case one
     /// is generated from the account tag's name.
     QColor color;
@@ -62,6 +74,13 @@ struct Account
 
     /// Restricts a notmuch query to this account's subtree.
     QString scopedQuery(const QString &query) const;
+
+    /// Matches this account's sent mail, or empty when `sent` is unset.
+    ///
+    /// Composes with scopedQuery() rather than replacing it: the account
+    /// selector wraps whatever query runs, so a Sent view under one account
+    /// intersects to that account's sent mail and cannot leak another's.
+    QString sentQuery() const;
 };
 
 struct SavedQuery
@@ -114,6 +133,18 @@ public:
     /// Optional alternate notmuch config file. Empty means "let notmuch decide".
     QString notmuchConfig() const { return m_notmuchConfig; }
 
+    /// Matches every configured account's sent mail, or empty when no account
+    /// configures one.
+    ///
+    /// Joins only the NON-EMPTY sentQuery() results. An account without a
+    /// `sent` key contributes nothing, and joining it anyway would leave a bare
+    /// `or` in the query. notmuch does not reject that: `A or  or B` returns
+    /// 190 messages where the correct pair returns 211, measured directly. A
+    /// malformed query that still returns plausible mail is the failure that
+    /// ships, which is why the join lives here and is tested rather than being
+    /// open-coded at the call site.
+    QString allSentQuery() const;
+
     /// A QDateTime::toString() pattern for the date on a card, or empty for the
     /// system locale's short format.
     ///