tui: pin read-tool images live in remote sessions

Remote/self-dev TUI sessions only populated the pinned-image side pane
(remote_side_pane_images) from full History reloads. When the read tool
returned an image mid-turn, the server sent ToolDone with text output
only, so the image never appeared in the side pane until the next
history reload.

Add ServerEvent::SidePaneImages, emitted from both streaming turn loops
when a tool output carries images, and handle it client-side by
appending to remote_side_pane_images and re-running the auto-hide reveal
logic. Includes diagnostic logging on both ends, protocol roundtrip
test, and client handler tests.