refactor(chat): replace timeline scroll engine with anchored-turn LegendList

Sending a message now parks that message near the top of the viewport
and streams the reply into reserved end space below it, instead of
jumping to the bottom and chasing it.

- swap @tanstack/react-virtual for @legendapp/list in the chat timeline; the
  streaming tail becomes a normal list row rather than a separately rendered
  block, so one component owns the scroll position
- add timelineScrollAnchoring: pure anchored-turn geometry plus the three
  scroll modes (following-end / anchoring-new-turn / free-scrolling)
- replace useChatAutoFollow with useChatTimelineScroll, which opts out of
  automatic movement on real gestures via a generation counter instead of the
  timer windows the old implementation needed to recognise its own writes
- move the load-older button, question/permission cards, recap, status row and
  bottom spacer into the list header/footer, since the list owns its container
- extract useScrollShadow so the shadows can attach to that container

maintainScrollAtEnd and maintainVisibleContentPosition replace the manual
prepend anchor-hold and the mobile quiet-window prepend deferral.

Validated: workspace type-check, lint, web build, ui tests per file.
Scroll behaviour itself is unverified and needs manual testing on web, desktop
and iOS.

packages/ui/src/components/chat/message/MessageBody.tsx

@@ -19,7 +19,7 @@ import { SaveProjectPlanDialog } from '@/components/session/SaveProjectPlanDialo
 import { ForkSessionDialog, type ForkSessionExecution } from '@/components/session/ForkSessionDialog';
 import { Tooltip, TooltipContent, TooltipTrigger } from '@/components/ui/tooltip';
 import { ArrowsMerge } from '@/components/icons/ArrowsMerge';
-import type { ContentChangeReason } from '@/hooks/useChatAutoFollow';
+import type { ContentChangeReason } from '@/hooks/useChatTimelineScroll';
 
 import { MarkdownImageGallery, SimpleMarkdownRenderer } from '../MarkdownRenderer';
 import { useSessionUIStore } from '@/sync/session-ui-store';