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/parts/ReasoningPart.tsx

@@ -2,7 +2,7 @@ import React from 'react';
 import { animate, type AnimationPlaybackControls } from 'motion';
 import type { Part } from '@opencode-ai/sdk/v2';
 import { cn } from '@/lib/utils';
-import type { ContentChangeReason } from '@/hooks/useChatAutoFollow';
+import type { ContentChangeReason } from '@/hooks/useChatTimelineScroll';
 import { ScrollableOverlay } from '@/components/ui/ScrollableOverlay';
 import { Icon } from '@/components/icon/Icon';
 import { BusyDots } from './BusyDots';