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/lib/api.ts

@@ -155,10 +155,10 @@ export function chatSync(
 }
 
 /** POST /api/v1/chat with stream:true — triggers streaming over the WS channel. */
-export function chatStream(sessionId: string, text: string, mode?: Mode): Promise<{ stream: true }> {
+export function chatStream(sessionId: string, text: string): Promise<{ stream: true }> {
   return request<{ stream: true }>("/chat", {
     method: "POST",
-    body: JSON.stringify({ sessionId, text, stream: true, mode }),
+    body: JSON.stringify({ sessionId, text, stream: true }),
   });
 }
 

frontend/web/src/lib/useChat.ts

@@ -2,19 +2,18 @@ import { useCallback, useState } from "react";
 import { chatStream } from "./api";
 import { connectWs, isWsOpen } from "./ws";
 import { useAppStore } from "../store/useAppStore";
-import { useModeStore } from "../store/useModeStore";
 import { now } from "./util";
-import type { Mode } from "./types";
 
 /** Drives one user turn: push the user bubble, blanket a streaming assistant
  *  bubble, trigger the backend loop over REST (`stream:true`), and let the WS
  *  stream render into that bubble. Falls back to the WS `chat` channel when the
  *  REST trigger fails so the demo still works without a wired loop.
  *
- *  Each message carries the current input mode, so its bubble renders per that
- *  mode (design/07 §1: shared message stream, per-message mode rendering). */
+ *  The behavior mode is NOT chosen by the user at send time — the agent / backend
+ *  hot-switches it (design/01 §3), and each message carries its own mode, so the
+ *  bubble renders per that message's mode. */
 export function useSendChat(): {
-  send: (text: string, mode?: Mode) => Promise<void>;
+  send: (text: string) => Promise<void>;
   sending: boolean;
 } {
   const [sending, setSending] = useState(false);
@@ -27,18 +26,17 @@ export function useSendChat(): {
   const appendTaskLog = useAppStore((s) => s.appendTaskLog);
 
   const send = useCallback(
-    async (text: string, mode?: Mode) => {
+    async (text: string) => {
       const trimmed = text.trim();
       if (!trimmed || sending) return;
       setSending(true);
       if (!isWsOpen()) connectWs();
 
-      const m = mode ?? "chat";
-      addUserMessage(trimmed, m);
-      beginAssistant(m);
+      addUserMessage(trimmed);
+      beginAssistant();
 
       try {
-        await chatStream(sessionId, trimmed, m);
+        await chatStream(sessionId, trimmed);
       } catch (err: unknown) {
         // Backend loop not wired or gateway unreachable: show the failure inline
         // rather than leaving a permanently-streaming bubble.