fix(chat): re-pin the thread when its scroll viewport resizes

use-stick-to-bottom only observes the transcript content, so a change in
the height of the scroll viewport itself went unnoticed. Shrinking it —
the live-turn stats bar mounting, a card opening in the composer dock,
or the terminal panel returning after a full-page route — moves the
bottom without moving scrollTop and fires neither a scroll event nor a
content resize, leaving the tail of the turn below the fold with the
scroll-to-bottom button hidden inside the library's 70px slack. Growing
it clamps scrollTop, which the library reads as the user scrolling up,
so the thread stops following the stream.

Observe the viewport and re-pin a thread that was already at the bottom,
marking the resize the way the library marks its own so the clamped
scroll can't escape the lock.