feat(ux): F3 流式进度感 + F4 友好错误文案（纯前端）

F4：新 lib/errors/messages.ts —— friendlyError(code, fallback) 把错误码映射为
中文文案 + 可选「去设置提供商」动作；覆盖后端 ErrorCode + 前端 NETWORK/
STREAM_FAILED；未知码回退原 message。写作页与审稿页错误块改用之，不再暴露
raw error code。+4 vitest。

F3：写作页工具条字数计数器流式时显示「生成中… N 字」+ 朱砂脉冲
（motion-safe:animate-pulse，尊重 prefers-reduced-motion），停后回「字数 N」。
纯 text.length，无后端改动。

前端门禁绿：lint/typecheck/vitest 171/build。无契约变更。

apps/web/components/review/ReviewReport.tsx

@@ -4,6 +4,7 @@ import { useEffect, useMemo, useRef, useState } from "react";
 
 import { AppShell } from "@/components/AppShell";
 import type { ProjectResponse, ReviewHistoryItem } from "@/lib/api/types";
+import { friendlyError } from "@/lib/errors/messages";
 import {
   allResolved,
   emptyDecisions,
@@ -226,21 +227,7 @@ export function ReviewReport({
           </div>
 
           {review.state.error ? (
-            <p className="border-b border-line bg-panel px-6 py-2 text-sm text-conflict">
-              审稿失败（{review.state.error.code}）：
-              {review.state.error.message}
-              {review.state.error.code === "LLM_UNAVAILABLE" ? (
-                <>
-                  {" "}
-                  <a
-                    href="/settings/providers"
-                    className="underline hover:text-cinnabar"
-                  >
-                    去设置提供商
-                  </a>
-                </>
-              ) : null}
-            </p>
+            <ReviewErrorNote error={review.state.error} />
           ) : null}
 
           <div className="flex-1 overflow-auto px-6 py-6">
@@ -346,6 +333,31 @@ export function ReviewReport({
   );
 }
 
+// 审稿失败提示：友好文案（不暴露 raw code）+ 可选「去设置」动作（F4）。
+function ReviewErrorNote({
+  error,
+}: {
+  error: { code: string; message: string };
+}) {
+  const friendly = friendlyError(error.code, error.message);
+  return (
+    <p className="border-b border-line bg-panel px-6 py-2 text-sm text-conflict">
+      {friendly.text}
+      {friendly.actionHref ? (
+        <>
+          {" "}
+          <a
+            href={friendly.actionHref}
+            className="underline hover:text-cinnabar"
+          >
+            {friendly.actionLabel}
+          </a>
+        </>
+      ) : null}
+    </p>
+  );
+}
+
 interface SectionStatusLineProps {
   reviewing: boolean;
   done: boolean;

apps/web/components/workbench/Workbench.tsx

@@ -5,6 +5,7 @@ import { useEffect, useRef, useState } from "react";
 
 import { AppShell } from "@/components/AppShell";
 import type { ProjectResponse } from "@/lib/api/types";
+import { friendlyError } from "@/lib/errors/messages";
 import { useAutosave } from "@/lib/autosave/useAutosave";
 import { useDraftStream } from "@/lib/stream/useDraftStream";
 import { WORKBENCH_CHAPTER_NO } from "@/lib/workbench/chapter";
@@ -93,6 +94,31 @@ export function Workbench({ project, initialText = "" }: WorkbenchProps) {
   );
 }
 
+// 写章失败提示：友好文案（不暴露 raw code）+ 可选「去设置」动作（F4）。
+function StreamErrorNote({
+  streamError,
+}: {
+  streamError: { code: string; message: string };
+}) {
+  const friendly = friendlyError(streamError.code, streamError.message);
+  return (
+    <p className="mb-2 text-sm text-conflict">
+      {friendly.text}
+      {friendly.actionHref ? (
+        <>
+          {" "}
+          <Link
+            href={friendly.actionHref}
+            className="underline hover:text-cinnabar"
+          >
+            {friendly.actionLabel}
+          </Link>
+        </>
+      ) : null}
+    </p>
+  );
+}
+
 interface ToolbarProps {
   projectId: string;
   chapterNo: number;
@@ -118,11 +144,7 @@ function Toolbar({
 }: ToolbarProps) {
   return (
     <div className="border-t border-line bg-panel px-6 py-3">
-      {streamError ? (
-        <p className="mb-2 text-sm text-conflict">
-          写章失败（{streamError.code}）：{streamError.message}
-        </p>
-      ) : null}
+      {streamError ? <StreamErrorNote streamError={streamError} /> : null}
       <div className="flex items-center gap-4">
         {streaming ? (
           <button
@@ -141,9 +163,15 @@ function Toolbar({
             ✍ 写本章
           </button>
         )}
-        <span className="font-mono text-xs text-ink-soft">
-          字数 {wordCount.toLocaleString("en-US")}
-        </span>
+        {streaming ? (
+          <span className="font-mono text-xs text-cinnabar motion-safe:animate-pulse">
+            生成中… {wordCount.toLocaleString("en-US")} 字
+          </span>
+        ) : (
+          <span className="font-mono text-xs text-ink-soft">
+            字数 {wordCount.toLocaleString("en-US")}
+          </span>
+        )}
         <Link
           href={`/projects/${projectId}/outline`}
           className="rounded border border-line px-4 py-2 text-sm text-ink hover:border-cinnabar hover:text-cinnabar"