Introduce ChatScrollController owning all chat scroll behavior:
- continuous bottom-follow intent tracked from LazyListState (not only on data change)
- pixel-precise bottom detection (last item edge vs viewport, small tolerance)
- streaming follow that pauses on upward scroll and never forces the reader down
- tail updates keyed on messages/streaming/thinking/subagent/clarify
- unread (new-message) count shown on the scroll-to-bottom FAB, cleared on tap
- paging anchor preserved by message id across older-history prepend
- all scroll commands (send/FAB/session switch/search/paging) serialized via one scope
Closes#682
Co-authored-by: M57 <hy4ri@users.noreply.github.com>
