feat(layout): 简洁模式右上角加换肤 + 界面模式下拉切换

抽出共享 appearance-controls（ThemeSwitcher 按 tone 适配 banner/content
配色，新增图标+下拉的 LayoutModeSwitcher）；Header 复用换肤组件，简洁模式
ChatPage 顶栏右上角补上换肤与界面模式切换，与全量模式保持一致。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/pages/ChatPage.tsx

@@ -26,6 +26,10 @@ import { ThreadContext } from "@/components/workspace/messages/context";
 import { QuestionNavigator } from "@/components/workspace/question-navigator";
 import { ThreadTitle } from "@/components/workspace/thread-title";
 import { TodoList } from "@/components/workspace/todo-list";
+import {
+  LayoutModeSwitcher,
+  ThemeSwitcher,
+} from "@/components/workspace/appearance-controls";
 import { TokenUsageIndicator } from "@/components/workspace/token-usage-indicator";
 import { UserIdentityBadge } from "@/components/workspace/user-identity-badge";
 import { Welcome } from "@/components/workspace/welcome";
@@ -484,8 +488,12 @@ export default function ChatPage() {
               <ArtifactTrigger />
               {/* 不带全局顶栏（紧凑模式）时，名称+岗位回落到聊天页顶栏右侧，
                   与全量模式 Header 右上角保持一致。全量模式已有顶栏则不重复展示。 */}
+              {/* 简洁模式无全局顶栏：把换肤 + 界面模式切换搬到页面级顶栏右上角，
+                  与全量模式 Header 右上角保持一致（content 配色贴浅底顶栏）。 */}
               {!compactChrome && (
                 <>
+                  <ThemeSwitcher tone="content" />
+                  <LayoutModeSwitcher tone="content" />
                   <div
                     className="bg-border/70 mx-1 h-6 w-px shrink-0"
                     aria-hidden