perf(chat): isolate the composer, split heavy chunks, fix dead tokens

Second batch from the staging UX audit (BUG-248..253).

- css: expose the 32 palette tokens through @theme. Seven utilities
  including text-ink, text-danger and text-warning compiled to no CSS
  at all, so 30 call sites had been silently inert (BUG-248)
- chat: move the composer into its own component behind a draft store,
  so a keystroke no longer re-renders a 2723-line component, and
  persist the draft across reloads (BUG-249)
- chat: load gsap, react-markdown and thinking-orbs on demand. First
  Load JS for / drops 549.5 kB to 476.3 kB gzipped (BUG-250)
- chat: route the five in-app destinations through router.push, and
  keep the five auth redirects and the bootstrap retry as hard loads
  on purpose (BUG-251)
- a11y: announce reply completion, and move the live region out of the
  aria-busy subtree that was likely suppressing even the start
  announcement (BUG-252)
- docs: give the 27 collided bug ids unique numbers and repair their
  inbound references; require search rather than a full read of a
  3690-line file (BUG-253)

Verified: tsc, eslint, next build, and 1592 assertions across the 199
non-database test files.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_bug_history_workflow.py

@@ -19,6 +19,7 @@ def test_agents_requires_read_before_and_update_after_bug_work() -> None:
     text = AGENTS.read_text(encoding="utf-8")
 
     assert "docs/BUG_HISTORY.md" in text
-    assert "开始诊断前必须完整读取并搜索" in text
+    assert "开始诊断前必须用报错原文、接口路径、状态码、模块名和用户操作强制检索" in text
+    assert "检索不是可选步骤" in text
     assert "必须在同一变更中更新" in text
     assert "严禁写入" in text