fix(ui): 加回 agent 头像，范围带调深到看得见

产品真机走查：

1. agent 头像加回来（推翻 R3 的 D7）。R3 删它的理由是「一屏几十条时是重复度
   最高的元素」，但没有它助手那一轮就没有视觉归属——生时校正面尤其明显，
   旁白、题干、选项全是平铺文字。--assistant-content-inset 随之回到
   「32px 头像 + 间隙」，追问 chip 与运行时间轴重新对齐正文。

2. 时间范围带用的是 --color-action-soft，那是给文字底色用的浅色调，
   画在画布上几乎看不见——而它是整条轴上唯一必须一眼看到的标记。
   改成 color-mix(--color-action-strong 32%, --color-canvas)。

测试 3369，fail 仍 31 且与基线逐条一致；四个路由标记不变。

未修：生时校正面顶部溢出。工作区在栅格里、没有脱离定位，
chat-panel.is-rectification 仍是 46px + 1fr 两行，从代码上看不出
46px 顶栏为什么会消失，待产品确认截图是否裁切。

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

frontend/src/components/chat-message-row.tsx

@@ -31,6 +31,14 @@ function motionPreferred() {
 
 if (motionPreferred()) prefetchOnIdle(loadGsap);
 
+/* Restored after real-device review: R3 removed it on the grounds that it was
+   the most repeated element in a long transcript, but without it the assistant
+   turn has no visual owner — in the rectification surface especially, where
+   narration, a question and its options all stack as plain text. */
+export function AgentAvatar() {
+  return <span className="agent-avatar" aria-hidden="true" />;
+}
+
 export const ChatMessageRow = memo(function ChatMessageRow({
   message,
   showActivity = message.state !== "settled",
@@ -123,6 +131,7 @@ export const ChatMessageRow = memo(function ChatMessageRow({
       className={`message message-${message.role}`}
       aria-label={message.role === "assistant" ? assistantLabel : "你"}
     >
+      {message.role === "assistant" && <AgentAvatar />}
       <div className="message-content">
         <div className="message-bubble">
           {message.role === "assistant" ? (

frontend/src/components/onboarding-chat-message.tsx

@@ -1,6 +1,7 @@
 "use client";
 
 import { ChatMessageContent } from "@/components/chat-message-content";
+import { AgentAvatar } from "@/components/chat-message-row";
 import { protectOnboardingPhrases } from "@/lib/onboarding-copy";
 import type { Message } from "@/lib/home-types";
 
@@ -9,6 +10,7 @@ export function OnboardingChatMessage({ role, text, streaming = false, length =
   const protectedVisibleText = protectOnboardingPhrases(visibleText);
   return (
     <article className={`message message-${role} onboarding-message${phraseSafe ? " is-phrase-safe" : ""}`} aria-label={role === "assistant" ? "Jyotisha" : "你"}>
+      {role === "assistant" && <AgentAvatar />}
       <div className="message-content">
         <div className="message-bubble">
           {role === "assistant" ? (