feat(frontend): 编辑器内「续写」——读本章前文生成多候选，点选插入正文

Phase 1（写作工作台重构）：编辑器「续写」按钮复用 continue 生成器（with_prior_chapter
自动读该章已写正文承接），每次产出一条候选并累加成多候选（useContinue，已单测），
「再来一个」可多生成几版；作者点某条「插入正文」才追加到章末（HITL，不静默写入）。
续写前 flush 自动保存，确保读到最新草稿。润色/续写结果卡互斥不并占。

apps/web/lib/workbench/useContinue.ts

@@ -0,0 +1,72 @@
+"use client";
+
+import { useCallback, useState } from "react";
+
+import { api } from "@/lib/api/client";
+import { useToast } from "@/components/Toast";
+import { generationErrorMessage } from "@/lib/generation/cards";
+
+// 编辑器「续写」：复用 continue 生成器（with_prior_chapter，自动读该章已写正文承接），
+// 每次生成一条候选并累加成多候选，作者点选某条插入正文（HITL，不静默写入）。
+export type ContinueStatus = "idle" | "generating" | "ready" | "error";
+
+export interface UseContinue {
+  status: ContinueStatus;
+  candidates: string[];
+  // 生成一条续写候选（累加，不覆盖）；chapterNo 指定承接哪一章。
+  generate: (projectId: string, chapterNo: number) => Promise<void>;
+  reset: () => void;
+}
+
+function extractText(preview: unknown): string {
+  if (preview && typeof preview === "object" && "text" in preview) {
+    const text = (preview as { text: unknown }).text;
+    return typeof text === "string" ? text : "";
+  }
+  return "";
+}
+
+export function useContinue(): UseContinue {
+  const [status, setStatus] = useState<ContinueStatus>("idle");
+  const [candidates, setCandidates] = useState<string[]>([]);
+  const toast = useToast();
+
+  const generate = useCallback<UseContinue["generate"]>(
+    async (projectId, chapterNo) => {
+      setStatus("generating");
+      try {
+        const { data, error } = await api.POST(
+          "/projects/{project_id}/skills/{tool_key}/generate",
+          {
+            params: { path: { project_id: projectId, tool_key: "continue" } },
+            body: { brief: "", chapter_no: chapterNo },
+          },
+        );
+        if (error || !data) {
+          setStatus("error");
+          toast(generationErrorMessage(error), "error");
+          return;
+        }
+        const text = extractText(data.preview).trim();
+        if (text.length === 0) {
+          setStatus("ready");
+          toast("本次未生成有效续写，请再试一次。", "info");
+          return;
+        }
+        setCandidates((prev) => [...prev, text]);
+        setStatus("ready");
+      } catch {
+        setStatus("error");
+        toast("续写请求异常，请检查网络。", "error");
+      }
+    },
+    [toast],
+  );
+
+  const reset = useCallback((): void => {
+    setStatus("idle");
+    setCandidates([]);
+  }, []);
+
+  return { status, candidates, generate, reset };
+}