feat(ui): render a single message when its row is selected

loadMessage queries by id: and returns one MessageRef, always matched, since the
user asked for that message by clicking its row and a stub would answer the
wrong question. An unknown id emits an empty vector rather than an error: a
stale row after a reindex is an ordinary race, not a failure worth the status
bar. The signal fires even when empty so the UI handler runs instead of waiting
for a reply that never comes.

The branch in onThreadSelected is placed BEFORE threadAt(), which is the whole
trap. threadAt takes a top-level row number and a child's row number indexes its
siblings, so handing a message row's number to it loads whichever thread happens
to sit at that position. Mutation-checked: with the branch disabled the test
reports thread 't1' for a reply belonging to 't2', a wrong answer plausible
enough to survive review.

m_currentMessageId and m_currentThreadId are mutually exclusive and each clears
the other, so a queued reply can tell which kind of selection it belongs to.
onMessageLoaded carries a third guard onThreadLoaded does not need: a reply
landing after the selection moved to a thread row would render one message where
the conversation belongs.

No mark-read timer for a message row in this pass. Marking one message of a
thread read is a per-message tag write and the pending-edit map is keyed by
thread; item 28 is the record of what happens when that count goes wrong.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/notmuchworker.h b/src/notmuchworker.h
index 99cad04..df1799d 100644
--- a/src/notmuchworker.h
+++ b/src/notmuchworker.h
@@ -70,6 +70,13 @@ public slots:
     void loadThreadTree(const QString &threadId, const QString &matchQuery,
                         quint64 generation);
 
+    /// Loads ONE message, for a message row selected in the list.
+    ///
+    /// Emits messageLoaded with an empty vector when the id is unknown, which
+    /// is an ordinary race after a reindex rather than an error worth
+    /// reporting.
+    void loadMessage(const QString &messageId, quint64 generation);
+
     /// Applies tag changes. Opens the database read-write, applies, and closes
     /// immediately: notmuch's write lock is exclusive process-wide, so holding
     /// it would block the user's cron `notmuch new`.
@@ -117,6 +124,7 @@ signals:
     void threadLoaded(const QVector<MessageRef> &messages, quint64 generation);
     void threadTreeLoaded(const QVector<MessageNode> &nodes,
                           quint64 generation);
+    void messageLoaded(const QVector<MessageRef> &messages, quint64 generation);
     void tagsApplied(const TagChange &change);
     void allTagsReady(const QStringList &tags, quint64 generation);