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.

bun.lock

@@ -167,6 +167,7 @@
         "@dnd-kit/core": "^6.3.1",
         "@dnd-kit/sortable": "^10.0.0",
         "@dnd-kit/utilities": "^3.2.2",
+        "@legendapp/list": "3.2.0",
         "@lezer/highlight": "^1.2.3",
         "@opencode-ai/sdk": "1.18.21",
         "@pierre/diffs": "1.3.0-beta.6",
@@ -917,6 +918,8 @@
 
     "@kwsites/promise-deferred": ["@kwsites/promise-deferred@1.1.1", "", {}, "sha512-GaHYm+c0O9MjZRu0ongGBRbinu8gVAMd2UZjji6jVmqKtZluZnptXGWhz1E8j8D2HJ3f/yMxKAUC0b+57wncIw=="],
 
+    "@legendapp/list": ["@legendapp/list@3.2.0", "", { "dependencies": { "use-sync-external-store": "^1.5.0" }, "peerDependencies": { "react": "*", "react-dom": "*", "react-native": "*" }, "optionalPeers": ["react-dom", "react-native"] }, "sha512-bN+g/oQYjFz+UAyuBN4cmYJAwdJS1TdNcZZOVlh3+VwCQUWrsg0PH46Mvm76gdZSCYMfoFanPY4dKnILcYEzeg=="],
+
     "@levischuck/tiny-cbor": ["@levischuck/tiny-cbor@0.2.11", "", {}, "sha512-llBRm4dT4Z89aRsm6u2oEZ8tfwL/2l6BwpZ7JcyieouniDECM5AqNgr/y08zalEIvW3RSK4upYyybDcmjXqAow=="],
 
     "@lezer/common": ["@lezer/common@1.5.1", "", {}, "sha512-6YRVG9vBkaY7p1IVxL4s44n5nUnaNnGM2/AckNgYOnxTG2kWh1vR8BMxPseWPjRNpb5VtXnMpeYAEAADoRV1Iw=="],