feat: IM平台列 + deviceID标识 + 会话持久化完善

- agent-96: chat_messages 加 platform 列(迁移0004) + IM主人私聊落主会话web(带platform) + 前端按平台过滤契约 + message_persisted 带 platform(复用 BroadcastPersisted)
- agent-97: chat_messages 加 device_id 列(迁移0005) + history.Message.DeviceID + Web发消息带deviceId(localStorage getClientId) + IM persistToMain 带 deviceID + 前端气泡展示 SourceLabel + 修主会话基础2个bug(fn scanMessage语法/DDL缺列)
- design/16 §4.4(平台) + §4.5(deviceID) 已入档
- 实机: POST /chat 带 deviceId 读回 user/assistant 均带 deviceId+platform; 后端 build/vet/test 绿 + 前端 build 绿
- 红线: 身份靠 actor 元数据; 禁止 shell 读 .env(JWT冒烟由主会话跑)

frontend/web/src/lib/useChat.ts

@@ -3,6 +3,7 @@ import { chatStream } from "./api";
 import { connectWs, isWsOpen } from "./ws";
 import { useAppStore } from "../store/useAppStore";
 import { useModeStore } from "../store/useModeStore";
+import { getClientId } from "./client";
 import { now } from "./util";
 
 /** Drives one user turn: push the user bubble, blanket a streaming assistant
@@ -38,8 +39,11 @@ export function useSendChat(): {
       // The gateway's chat body still requires a non-empty text, so a pure image
       // turn (vision) falls back to a default prompt to keep the trigger valid.
       const prompt = trimmed || (image ? "分析这张图片" : "");
-      addUserMessage(trimmed, undefined, image);
-      beginAssistant(mode);
+      // Tag this browser's persistent client id onto the optimistic bubbles so the
+      // source label shows immediately and matches the persisted turn (design/16 §4.5).
+      const clientId = getClientId();
+      addUserMessage(trimmed, undefined, image, clientId);
+      beginAssistant(mode, clientId);
 
       try {
         await chatStream(sessionId, prompt, image);