feat(web): 聊天请求携带行为模式 mode(A3) — useChat 读当前 mode 经 REST 发送

- api.ts chatStream 新增可选 mode，空则不写键(后端回落chat)
- useChat.ts 发送时带上 useModeStore 当前 mode，使非chat模式消息在Web端真正落库
- 修正 docblock(移除虚构的WS chat回退描述)

frontend/web/src/lib/useChat.ts

@@ -8,13 +8,13 @@ import { now } from "./util";
 
 /** 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.
+ *  stream render into that bubble. If the REST trigger fails, the failure is
+ *  surfaced inline on the bubble rather than left streaming forever.
  *
- *  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. Until the backend advertises a
- *  per-message mode, a new assistant bubble defaults to the current UI mode. */
+ *  The behavior mode is chosen by the agent / backend hot-switch (design/01 §3);
+ *  each message carries the mode in effect when the turn is sent, and the bubble
+ *  renders per that message's mode. We read the current UI mode here and pass it
+ *  on the turn so the backend persists it alongside the message (A3). */
 export function useSendChat(): {
   send: (text: string, image?: string) => Promise<void>;
   sending: boolean;
@@ -46,7 +46,7 @@ export function useSendChat(): {
       beginAssistant(mode, clientId);
 
       try {
-        await chatStream(sessionId, prompt, image);
+        await chatStream(sessionId, prompt, image, mode);
       } catch (err: unknown) {
         // Backend loop not wired or gateway unreachable: show the failure inline
         // rather than leaving a permanently-streaming bubble.