tui: stop transcript scroll from accumulating phantom offset

Scrolling the chat down while already pinned to the bottom previously kept
'succeeding' for every queued mouse-wheel/momentum step, draining the
mouse-scroll queue slowly without moving the view. That left phantom scroll
that a later scroll-up had to undo before the viewport moved again.

scroll_up/scroll_down now report whether the stored position actually
changed; the mouse-scroll step uses that to clear the queue once a step is
blocked at the boundary. Also cap the paused offset at the renderer's exact
extent and treat a non-streaming, non-scrollable transcript as already at
the bottom.