refactor(chat): extract scroll lifecycle into useScrollLifecycle hook

- Extract ~400 lines of scroll lifecycle logic from ChatView.tsx into
  a dedicated useScrollLifecycle hook, reducing ChatView scroll-related
  refs from ~17 to 0 and making the logic testable in isolation.

- Reduce INITIAL_LOAD_SETTLE_HARD_CAP_MS from 10s to 5s. If rehydration
  takes longer, there is likely a rendering performance issue worth
  investigating separately.

- Document the scrollToIndex reversal: PR #6780 removed scrollToIndex
  due to jitter from stale numeric indices. The "LAST" constant used
  here resolves at call time, avoiding that issue.

- All 6 existing scroll regression tests pass unchanged.

Author: roomote