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/tests/chat-stream-layout.test.ts

@@ -152,7 +152,10 @@ test("nothing sits between the transcript and the composer to shift height while
     pageSource.indexOf("<ChatComposer"));
 
   assert.doesNotMatch(composerWrap, /composer-suggestions/);
-  assert.match(composerWrap, /pointer-events-none absolute inset-x-0 bottom-full/);
+  // Former lock: the inline `pointer-events-none absolute inset-x-0 bottom-full` wrapper. The
+  // control is now the shared JumpToLatestButton, positioned by `.jump-to-latest` (BUG-478).
+  assert.match(composerWrap, /<JumpToLatestButton/);
+  assert.match(globalStyles, /\.jump-to-latest \{[^}]*position: absolute[^}]*bottom: 100%/);
 });
 
 test("docks the composer inside the chat panel instead of floating over content", () => {