fix(tui): keep Alt+M hidden image side panel hidden across server reload

The pinned image auto-hide logic re-revealed the panel whenever the seen
image count grew. On a server reload/reconnect, pinned_images_seen_count
resets to 0 while remote_side_pane_images is repopulated from the history
snapshot, which looked like new images and undid an explicit Alt+M hide.

Track an explicit-hide flag so a deliberate Alt+M close stays sticky and
is not overridden by transient image repopulation.