refactor(web): ChatView's timeline follow and anchoring lives in useTimelineFollow

The scroll-follow state, its refs, the scroll listeners and the new-turn
anchor were ~550 lines in the middle of ChatView. The two send paths arm the
anchor through one anchorNewTurn call instead of repeating nine statements.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
c61cf64daeJulius Marminge committed on 10/21/2026, 7:46:49 AMparent9c5f157

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