feat(frontend): 五类交互接入 AI 对话留痕 + Workbench 抽屉入口（AC-3）

- RefinePanel/ChapterRewritePanel：结束后 append [author(意见/答复), ai(产出)]；
  clarify buffer-until-concluded——澄清 Q&A（原意见→反问→答复）缓冲，随最终产出批
  一起落库同 thread；纯放弃不记。refine ai 行 meta 存 version_no+原选段（每版重锚）。
- ContinuePanel：ai-only（续写无作者输入），每候选一条 ai 气泡 meta{candidate_index}。
- GeneratorRunner：成功后 append [author(字段摘要, meta{tool_key,input_fields}),
  ai(渲染预览, meta{tool_key,output_kind})]；空产物不记。lib/toolbox/aiSummary.ts 纯助手+单测。
- InlineToolbox→GeneratorRunner 透传 append+本章号；ToolboxPage 用 silent 直投项目级(chapter_no=null)。
- Workbench：挂 useAiConversation 单实例 + AiConversationDrawer + 底栏「AI 对话」入口（flag 灰度），
  三个 DRAFT-only 再接受回调（替换整章/插入正文/回填选段），成功关抽屉+toast、漂移不盲替换。

apps/web/components/workbench/ContinuePanel.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useEffect, useRef } from "react";
+import { useCallback, useEffect, useRef } from "react";
 import { Check, Plus, X } from "lucide-react";
 
 import { ThinkingIndicator } from "@/components/ThinkingIndicator";
@@ -8,12 +8,15 @@ import { Button } from "@/components/ui/Button";
 import { SectionHeader } from "@/components/ui/SectionHeader";
 import { StatusNote } from "@/components/ui/StatusNote";
 import { useContinue } from "@/lib/workbench/useContinue";
+import type { AiTurnInput } from "@/lib/workbench/useAiConversation";
 
 interface ContinuePanelProps {
   projectId: string;
   chapterNo: number;
   // 插入选中候选到正文（追加到章末）。
   onInsert: (text: string) => void;
+  // 留痕：每条候选记一条 ai 气泡（续写无作者输入，故无 author 气泡）。
+  appendMessages: (turn: AiTurnInput) => void;
   onClose: () => void;
 }
 
@@ -23,16 +26,37 @@ export function ContinuePanel({
   projectId,
   chapterNo,
   onInsert,
+  appendMessages,
   onClose,
 }: ContinuePanelProps) {
   const { status, candidates, generate } = useContinue();
   const ranRef = useRef(false);
+  // 本面板一次会话 = 一个 thread；多条候选归此组，candidate_index 本地累加。
+  const threadId = useRef(crypto.randomUUID()).current;
+  const candidateIdxRef = useRef(0);
+
+  // 生成一条候选并（成功时）留痕：ai-only，meta 存候选序号。
+  const runGenerate = useCallback(async (): Promise<void> => {
+    const text = await generate(projectId, chapterNo);
+    if (!text) return;
+    const candidateIndex = candidateIdxRef.current;
+    candidateIdxRef.current += 1;
+    appendMessages({
+      threadId,
+      kind: "continue",
+      toolKey: "continue",
+      chapterNo,
+      messages: [
+        { role: "ai", content: text, meta: { candidate_index: candidateIndex } },
+      ],
+    });
+  }, [generate, projectId, chapterNo, appendMessages, threadId]);
 
   useEffect(() => {
     if (ranRef.current) return;
     ranRef.current = true;
-    void generate(projectId, chapterNo);
-  }, [projectId, chapterNo, generate]);
+    void runGenerate();
+  }, [runGenerate]);
 
   const busy = status === "generating";
 
@@ -58,7 +82,7 @@ export function ContinuePanel({
         <StatusNote variant="danger" className="mt-3 text-xs" title="续写失败">
           <button
             type="button"
-            onClick={() => void generate(projectId, chapterNo)}
+            onClick={() => void runGenerate()}
             className="mt-1 inline-flex items-center gap-1 text-cinnabar hover:underline"
           >
             重试
@@ -90,7 +114,7 @@ export function ContinuePanel({
 
       <div className="mt-3">
         <Button
-          onClick={() => void generate(projectId, chapterNo)}
+          onClick={() => void runGenerate()}
           disabled={busy}
           variant="secondary"
           size="sm"