fix(read): repaint the card when one message is marked read

Follows the message-scoped mark-read. The user reported the write going
out with nothing visible changing: the status bar counted an unsynced
edit while the card stayed bold and the message pane still showed the
`unread` tag, until the next query corrected it.

sendMessageTagChange made no optimistic model update on purpose, because
applyTagChange is keyed by THREAD and repainting a whole row for a
one-message edit would claim every reply had changed too. That trade is
right for an explicit tag edit and wrong for auto mark-read, where the
visible change IS the feature and the delay exists to deliver it.

ThreadListModel::applyMessageTagChange updates the message wherever it
is held, as a child row and as `first`, and lets the thread's summary
follow only when the answer is unambiguous: a thread reads as unread
while ANY message does, so the tag is cleared from the thread only when
no other message still carries it. For an unexpanded multi-message
thread the per-message tags are not loaded, so the summary is left for
the next query rather than guessed at.

Mutation checked: without the call the card holds `unread` for the full
timeout.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/threadlistmodel.h b/src/threadlistmodel.h
index 2b8d2b0..f7abaf8 100644
--- a/src/threadlistmodel.h
+++ b/src/threadlistmodel.h
@@ -273,6 +273,23 @@ public:
     void applyTagChange(const QString &threadId, const QStringList &added,
                         const QStringList &removed);
 
+    /// The same, scoped to ONE message.
+    ///
+    /// Updates that message's own tags wherever it is held: as a child row if
+    /// the thread is expanded, and as `first` when it is the thread's opening
+    /// message. The thread's summary tags follow only when the change is
+    /// unambiguous for the whole thread, which for `unread` means no other
+    /// message still carries it, since a thread reads as unread while any of
+    /// its messages does.
+    ///
+    /// Exists because auto mark-read touches one message and the card still
+    /// has to stop looking unread. applyTagChange() above cannot serve that:
+    /// it rewrites the thread's tags directly, which for a multi-message
+    /// thread would claim every reply had been read.
+    void applyMessageTagChange(const QString &messageId,
+                               const QStringList &added,
+                               const QStringList &removed);
+
 private:
     /// One thread root and the message rows expanded under it.
     ///