refactor(chat): share composer, scroll follow and jump control across both chat surfaces

The rectification session rendered its own textarea (no length ceiling,
no count), its own rAF scroll follow with a sticky-scroll helper, and its
own jump chip styled differently from the main chat's inline Tailwind
button, while the main chat re-ran scrollTo on every streamed token and
added a smooth scroll on settle. ChatComposer now accepts a controlled
value so the rectification surface reuses it without touching the main
draft store; useConversationScrollAnchor owns the follow through a
ResizeObserver, one frame per content change, for both surfaces; a
single JumpToLatestButton replaces both chips; the 720px transcript
override is gone. DESIGN.md records the shared composer, the jump
control, the reading width and the two shadow tokens.

BUG-477 BUG-478

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JUei7K13cYxLHE3Axe4A45

frontend/src/lib/rectification-sticky-scroll.ts

@@ -1,43 +0,0 @@
-/**
- * Sticky follow for a streaming message viewport.
- *
- * Auto-follow only when the user is already near the bottom. Streaming
- * updates must not steal scrollTop after the user moves upward.
- */
-
-export const RECTIFICATION_NEAR_BOTTOM_PX = 96;
-/** Keep the jump chip hidden while a choice card still occupies the composer overlay band. */
-export const RECTIFICATION_CHOICE_NEAR_BOTTOM_PX = 360;
-
-export function distanceFromBottom(
-  scrollHeight: number,
-  scrollTop: number,
-  clientHeight: number,
-): number {
-  return scrollHeight - scrollTop - clientHeight;
-}
-
-export function isNearBottom(
-  scrollHeight: number,
-  scrollTop: number,
-  clientHeight: number,
-  thresholdPx = RECTIFICATION_NEAR_BOTTOM_PX,
-): boolean {
-  return distanceFromBottom(scrollHeight, scrollTop, clientHeight) <= thresholdPx;
-}
-
-export function shouldShowJumpToLatest(
-  scrollHeight: number,
-  scrollTop: number,
-  clientHeight: number,
-  choiceCardOpen = false,
-): boolean {
-  const distance = distanceFromBottom(scrollHeight, scrollTop, clientHeight);
-  if (distance <= RECTIFICATION_NEAR_BOTTOM_PX) return false;
-  if (choiceCardOpen && distance <= RECTIFICATION_CHOICE_NEAR_BOTTOM_PX) return false;
-  return true;
-}
-
-export function shouldFollowLatest(followTail: boolean): boolean {
-  return followTail === true;
-}