feat(ui): 强化 AI 工作中动效——构思占位/流式进度条/波动三点/转圈

- 动效基元：ai-dot 波动(三点竖跳)、ai-stream 不确定进度条、ai-breathe 呼吸微光（均带 reduced-motion 回退）
- ThinkingIndicator 升级为明显波动；新增 Spinner / StreamingBar / GenerationSkeleton 原子件；Button 加 loading 态
- 写作台：首 token 前正文区『AI 正在构思本章…』占位（补最大缺口，不再空白像卡死）+ 流式顶部进度条
- 整章重写流式进度条；续写/润色忙碌骨架 + 触发键转圈；生成器/角色/世界观改用 Button loading

apps/web/components/workbench/Workbench.tsx

@@ -6,6 +6,7 @@ import { AppShell } from "@/components/AppShell";
 import { Drawer } from "@/components/Drawer";
 import { useToast } from "@/components/Toast";
 import { StatusNote } from "@/components/ui/StatusNote";
+import { StreamingBar } from "@/components/ui/StreamingBar";
 import type { ProjectResponse } from "@/lib/api/types";
 import { friendlyError } from "@/lib/errors/messages";
 import { useAutosave } from "@/lib/autosave/useAutosave";
@@ -90,6 +91,8 @@ export function Workbench({
   const toast = useToast();
   const autosave = useAutosave(project.id, chapterNo, initialText);
   const stream = useDraftStream();
+  // 构思中 = 流式已开始但首 token 未到（正文区覆盖构思占位）；有字后即为在吐字。
+  const isThinking = stream.isStreaming && stream.state.text.length === 0;
   const lastStreamText = useRef("");
   const canRefine = selection !== null && selection.text.trim().length > 0;
   const chapterTriggerRef = useRef<HTMLButtonElement>(null);
@@ -382,6 +385,10 @@ export function Workbench({
               onClose={() => setRewriteOpen(false)}
             />
           ) : null}
+          {/* 写本章流式期间：编辑区顶部常挂一条不确定进度条，明确「进行中」（生成结束卸载）。 */}
+          {stream.isStreaming ? (
+            <StreamingBar label="AI 正在写本章" className="shrink-0" />
+          ) : null}
           <div ref={editorScrollRef} className="flex-1 overflow-auto px-6 py-8">
             {chapters.length === 0 ? (
               <StatusNote variant="info" className="mb-4">
@@ -392,6 +399,7 @@ export function Workbench({
               value={text}
               onChange={onEditorChange}
               streaming={stream.isStreaming}
+              isThinking={isThinking}
               onSelectionChange={setSelection}
             />
           </div>