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, 9 insertions, 0 deletions

diff --git a/src/mainwindow.h b/src/mainwindow.h
index fc980a0..5b7e39e 100644
--- a/src/mainwindow.h
+++ b/src/mainwindow.h
@@ -171,6 +171,10 @@ private slots:
     /// Fills in the expanded thread's message rows.
     void onThreadTreeLoaded(const QVector<MessageNode> &nodes,
                             quint64 generation);
+
+    /// Renders the single message a message row asked for.
+    void onMessageLoaded(const QVector<MessageRef> &messages,
+                         quint64 generation);
     void onWorkerError(const QString &message);
     void onSyncFinished(bool success, int exitCode);
 
@@ -503,6 +507,11 @@ private:
     QString m_lastQuery;
     QString m_currentThreadId;
 
+    /// The message a MESSAGE row is showing, empty whenever the pane holds a
+    /// whole thread. The two are mutually exclusive and each clears the other,
+    /// so a late reply can tell which kind of selection it belongs to.
+    QString m_currentMessageId;
+
     /// The selection count last written to the status bar, so it can be taken
     /// back without clobbering a message some other action put there.
     QString m_selectionMessage;