fix(rectification): 顶栏被 focus 滚出视野——面板改 clip，面板内 focus 一律 preventScroll

产品实测：生时校正答了三题后 46px 顶栏消失。devtools 取证——栅格是对的
（rows "46px 1297px"、panel y=0），但 header 自己的 rect 在 **y=-88**：
面板被程序化滚动了 88px。

根因：`overflow: hidden` 仍然是滚动容器，它只是去掉了滚动条。
BirthTimeChoiceQuestion 在每答完一题后 focus 新题的第一个选项，浏览器
为把它带进视野会滚动所有可滚动祖先，`.chat-panel` 就是其中之一——
而用户没有滚动条可以滚回来，顶栏于是永久消失。

两处都修：
- 病因：面板内 7 处程序化 focus 一律加 { preventScroll: true }。
  消息区有自己的 useConversationScrollAnchor，本来就不需要浏览器代劳。
  账户弹窗的 closeButton / returnTarget 不在此列——那是对话框焦点管理。
  同一教训 use-billing-panel.ts 已经吃过一次（那里早写了 preventScroll）。
- 结构：.chat-panel 与 .chat-app 从 overflow:hidden 改成 overflow:clip。
  clip 根本不创建滚动容器，此后任何 focus / scrollIntoView 都无法位移它。

新增 tests/chat-panel-scroll-guard.test.ts：锁住两个容器必须是 clip、
面板内不得有裸 focus()，并遍历 rectification/birth-time/chat- 全部组件，
新组件再写裸 focus 会直接打红。

测试 3372（+3），fail 仍 31 且与基线逐条一致；四个路由标记不变。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0193vBv6w5MV2cifdTUu9H5P

frontend/src/components/birth-time-choice-question.tsx

@@ -48,7 +48,10 @@ export function BirthTimeChoiceQuestion(props: ChoiceQuestionProps) {
   const firstChoiceRef = useRef<HTMLButtonElement | null>(null);
   useEffect(() => {
     if (!props.pending && (props.progress.answeredCount > 0 || props.error)) {
-      firstChoiceRef.current?.focus();
+      /* preventScroll: the transcript has its own anchor; letting the browser
+         scroll ancestors here scrolls `.chat-panel`, an overflow:hidden box with
+         no scrollbar, and the 46px header never comes back. */
+      firstChoiceRef.current?.focus({ preventScroll: true });
     }
   }, [props.error, props.pending, props.progress.answeredCount, props.question.questionId]);
   const select = (option: PublicDynamicChoiceQuestion["options"][number]) => {
@@ -103,7 +106,7 @@ export function BirthTimeUnmatchedClarification(props: ClarificationProps) {
   const reframeRef = useRef<HTMLButtonElement | null>(null);
   useEffect(() => {
     if (!props.pending && (props.progress.answeredCount > 0 || props.error)) {
-      reframeRef.current?.focus();
+      reframeRef.current?.focus({ preventScroll: true });
     }
   }, [props.error, props.pending, props.progress.answeredCount]);
   return (

frontend/src/components/birth-time-rectification.tsx

@@ -59,7 +59,7 @@ export function BirthTimeRectification(props: BirthTimeRectificationProps) {
     if (!changed || props.journey.journeyProtocol !== "dynamic-choice-v2") return;
     const action = props.journey.nextAction;
     if (action.kind !== "ask_dynamic_choice" && action.kind !== "clarify_unmatched_answer") {
-      assessmentHeadingRef.current?.focus();
+      assessmentHeadingRef.current?.focus({ preventScroll: true });
     }
   }, [props.journey]);
   if (props.journey.journeyProtocol !== "dynamic-choice-v2") {

frontend/src/components/rectification-agentic-chat.tsx

@@ -1824,7 +1824,7 @@ export function RectificationAgenticChat(props: RectificationAgenticChatProps) {
               const recalled = queued.take();
               if (!recalled) return;
               setDraft(appendQueuedText(recalled, draft));
-              composer.current?.focus();
+              composer.current?.focus({ preventScroll: true });
             } } : undefined}
             onSubmit={submit}
             onChange={(event) => setDraft(event.target.value)}

frontend/src/components/rectification-board.tsx

@@ -273,7 +273,10 @@ export function RectificationBoard({
 
   useEffect(() => {
     if (!compact || !open) return;
-    sheetRef.current?.focus();
+    /* preventScroll for the same reason as the choice options: this sheet
+       lives in the panel's grid cell, and the browser would scroll the
+       panel to reach it. */
+    sheetRef.current?.focus({ preventScroll: true });
     const handleKey = (event: KeyboardEvent) => {
       if (event.key === "Escape") onClose();
     };