Merge branch 'compose-and-send': composing and sending mail

Item 123, built over 2026-08-20 to 2026-08-23 in thirteen tasks against
docs/superpowers/specs/2026-08-20-compose-and-send-design.md.

The application writes mail now. A composer window per message, markdown as
the body, drafts autosaving into the account's Maildir, and sending through a
per-account command on stdin rather than any network protocol of this
program's own. A countdown with an Undo stands between pressing Send and the
command running.

Two things came in alongside it. The notmuch auto-tagging hooks moved here
from the retiring `mailctl` project and learned that mail this application
files itself never arrived, so sent mail and drafts stop appearing in the
inbox. And the v1/v2 language is retired: semver on the user-visible surface
is the rule, and those labels described a split that composing made obsolete.

Hand tested against a fake send command rather than a real one, deliberately:
New, Reply and Forward all produce correct messages, a forwarded attachment
survives intact, and the sent copy is filed. That testing found the two
defects fixed on this branch, and both were invisible to the suite: a composer
orphaned by quitting the main window, and every sent message tagged `inbox`.

Twenty-two defects were found in the plan document's own draft code while
building it, which is why CLAUDE.md says to treat every code block in a plan
as a draft.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/messageview.h b/src/messageview.h
index 3cc1604..044bded 100644
--- a/src/messageview.h
+++ b/src/messageview.h
@@ -128,6 +128,15 @@ public:
     /// Tags of the thread on display, shown as chips along the bottom.
     void setTags(const QStringList &tags);
 
+    /// Shows or hides the receive-only explanation, naming \p accountKey.
+    /// An empty key hides it.
+    ///
+    /// A WIDGET in this layout, never markup inside the web view. Composing
+    /// HTML from configuration into the one document that renders input from
+    /// strangers is the wrong direction, and the header row is already a
+    /// widget for the same reason.
+    void setReceiveOnlyAccount(const QString &accountKey);
+
     /// The full headers of every message in the thread, read-only. Also
     /// reachable from the button beside the header; public so the window's
     /// message_details action can call it.
@@ -391,6 +400,7 @@ private:
 
     QLabel *m_headerLabel = nullptr;
     QLabel *m_blockedLabel = nullptr;
+    QLabel *m_receiveOnlyRibbon = nullptr;
     QPushButton *m_loadRemoteButton = nullptr;
 
     /// The stale-thread notice and the thread it offers to restore.