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, 59 insertions, 0 deletions

diff --git a/src/messagebuilder.h b/src/messagebuilder.h
new file mode 100644
index 0000000..f9de277
--- /dev/null
+++ b/src/messagebuilder.h
@@ -0,0 +1,59 @@
+/*
+ * qtmaildir - a Qt6 mail client for notmuch-indexed Maildirs
+ * Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#pragma once
+
+#include <QByteArray>
+#include <QString>
+
+#include "types.h"
+
+struct Account;
+
+/// Turns an OutgoingMessage into the RFC822 bytes that get sent.
+///
+/// ONE built message serves three consumers: the autosaved draft, the bytes on
+/// the send command's stdin, and the sent copy. A draft is therefore
+/// byte-identical to what would be sent.
+///
+/// GMime rather than assembling RFC822 by string. The alternative means
+/// reimplementing RFC 2047 header encoding, quoted-printable for accented
+/// bodies, boundary uniqueness and line-length limits. This user writes
+/// Italian; a body containing an accented character is every message, and a
+/// bug there produces mail that looks correct locally and arrives as mojibake.
+namespace MessageBuilder {
+
+struct Result
+{
+    QByteArray bytes;    ///< The complete message. Empty on failure.
+    QString error;       ///< Empty on success.
+    QString messageId;   ///< The generated Message-ID, for the caller's records.
+
+    bool ok() const { return error.isEmpty(); }
+};
+
+/// Builds \p message as sent from \p account.
+///
+/// Fails, rather than sending a partial message, when an attachment named in
+/// the message no longer exists. That is checked HERE, at build time, rather
+/// than when the file was attached: a file can vanish in between, and the
+/// failure must stop the send rather than produce a message missing the thing
+/// it was written to carry.
+Result build(const OutgoingMessage &message, const Account &account);
+
+}  // namespace MessageBuilder