refactor(web): 前端单页收敛修正——导航式侧边栏 + 去输入模式 chip，mode 由后端自切

按设计/01 §3 + 主人要求：行为模式切换由后端/agent 自切（热切换），前端不再提供
手动切换的行为模式按钮列表。
- 侧边栏改为导航式（连接状态 / 设置 / 管理面板 / 主题），不再列行为模式按钮；
  App.tsx 还原 <Sidebar/> 布局。
- ChatInput 去掉行为模式 chip 切换条（模式不由用户手选）。
- useChat.send/chatStream 不再传 mode；消息 mode 由后端在回复时决定，
  MessageBubble 仍按 message.mode 渲染。
- ChatShell onVoice 为占位（语音处理在 agent）。

frontend/web/src/modes/ChatShell.tsx

@@ -1,6 +1,5 @@
 import { useEffect, useRef } from "react";
 import { useAppStore } from "../store/useAppStore";
-import { useModeStore } from "../store/useModeStore";
 import { useSendChat } from "../lib/useChat";
 import { MessageBubble } from "../components/chat";
 import { ChatInput } from "../components/ChatInput";
@@ -9,15 +8,14 @@ import { Icon } from "../components/Icon";
 
 /**
  * The single shared page (design/07 §1). All behavior modes live in ONE message
- * stream; each message carries its own `mode`, so its bubble renders per that
- * mode — a session can mix chat / code / home / docs / work / vision shapes.
- * The "input mode" chip above the composer sets the mode of the NEXT message.
+ * stream; each message carries its own `mode` (assigned by the agent at runtime,
+ * design/01 §3 hot-switch), so its bubble renders per that mode. There is no
+ * manual mode selector — the agent / backend drives the mode.
  */
 export function ChatShell(): React.ReactNode {
   const messages = useAppStore((s) => s.messages);
   const { send, sending } = useSendChat();
   const bottomRef = useRef<HTMLDivElement>(null);
-  const setMode = useModeStore((s) => s.setMode);
 
   useEffect(() => {
     bottomRef.current?.scrollIntoView({ behavior: "smooth", block: "end" });
@@ -39,7 +37,7 @@ export function ChatShell(): React.ReactNode {
           <div ref={bottomRef} />
         </div>
       </div>
-      <ChatInput onSend={(t, mode) => void send(t, mode)} sending={sending} onVoice={() => setMode("voice")} />
+      <ChatInput onSend={(t) => void send(t)} sending={sending} onVoice={() => { /* voice handled by agent */ }} />
     </div>
   );
 }