feat(web): 前端收敛为「共享消息流 + 按消息 mode 渲染」，并补全 emoji→MD3 图标

架构变更（主人要求，design/07 §1/§2 已改定稿）：把「9 个行为模式 = 9 个独立页面」改为
「一个共享页面 + 每条消息绑 mode，消息框按对应 mode 渲染」。
- ChatMessage 增 mode?（types.ts）；useAppStore addUserMessage/beginAssistant 带 mode；
  useSendChat.send(text, mode) / chatStream body 带 mode（通知后端按 mode 回复）。
- MessageBubble 按 message.mode 分发到 components/messages/ 各形态渲染器
  （chat 气泡 / code 等宽代码块 / home 设备卡 / docs 文档卡 / work 任务卡 / vision 视觉卡）。
- ChatInput 顶部加「行为模式」chip 切换下一条消息的 mode（默认 chat）；输入框上方显示当前模式。
- ModeRouter 收敛为只渲染 ChatShell；删除 9 个旧独立 mode 页面；Nav/TopBar 的 mode 改为
  「输入模式」语义；settings 归管理面板（design/10）。

同时补全上一轮 emoji→MD3 图标（约 23 文件）：新增 Icon.tsx（MD3 SVG、IconName、零依赖、
离线优先），全部界面 emoji 替换为 <Icon name=... />；正文说明箭头保留。

npm run build 绿（tsc 0 错，79→71 模块，JS 284KB/gzip 86KB）。

frontend/web/src/modes/ModeRouter.tsx

@@ -1,38 +1,12 @@
-import { useModeStore } from "../store/useModeStore";
-import { ChatMode } from "./ChatMode";
-import { CodeMode } from "./CodeMode";
-import { HomeMode } from "./HomeMode";
-import { WorkMode } from "./WorkMode";
-import { DocsMode } from "./DocsMode";
-import { VoiceMode } from "./VoiceMode";
-import { VisionMode } from "./VisionMode";
-import { AgentsMode } from "./AgentsMode";
-import { SettingsMode } from "./SettingsMode";
+import { ChatShell } from "./ChatShell";
 
-/** Renders the active mode's presentation layer. The session context / messages
- *  / memory live in useAppStore and are untouched by these switches. */
+/**
+ * User frontend is ONE shared page (design/07 §1): behavior modes no longer
+ * switch between nine distinct pages. Instead there is a single message stream
+ * and each message carries its own mode, so its bubble renders per that mode.
+ * Switching the input mode only shapes the next message. Settings live in the
+ * admin panel (design/10); agent progress follows the message stream.
+ */
 export function ModeRouter(): React.ReactNode {
-  const mode = useModeStore((s) => s.mode);
-  switch (mode) {
-    case "chat":
-      return <ChatMode />;
-    case "code":
-      return <CodeMode />;
-    case "home":
-      return <HomeMode />;
-    case "work":
-      return <WorkMode />;
-    case "docs":
-      return <DocsMode />;
-    case "voice":
-      return <VoiceMode />;
-    case "vision":
-      return <VisionMode />;
-    case "agents":
-      return <AgentsMode />;
-    case "settings":
-      return <SettingsMode />;
-    default:
-      return <ChatMode />;
-  }
+  return <ChatShell />;
 }