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/store/useAppStore.ts

@@ -66,8 +66,8 @@ export interface AppStore {
    *  backend id) so a message never renders twice (design/16 §4.3 dedup). */
   appendOrUpdateMessage: (msg: ChatMessage) => void;
 
-  addUserMessage: (text: string, mode?: Mode, image?: string) => void;
-  beginAssistant: (mode?: Mode) => string;
+  addUserMessage: (text: string, mode?: Mode, image?: string, deviceId?: string) => void;
+  beginAssistant: (mode?: Mode, deviceId?: string) => string;
   appendChunk: (delta: string) => void;
   endStream: () => void;
   setThinking: (text: string) => void;
@@ -172,6 +172,8 @@ export const useAppStore = create<AppStore>()(
               mode: msg.mode ?? m.mode,
               mood: msg.mood ?? m.mood,
               image: msg.image ?? m.image,
+              platform: msg.platform ?? m.platform,
+              deviceId: msg.deviceId ?? m.deviceId,
               createdAt: msg.createdAt ?? m.createdAt,
             };
             return { messages };
@@ -181,7 +183,7 @@ export const useAppStore = create<AppStore>()(
           return { messages: [...s.messages, msg] };
         }),
 
-      addUserMessage: (text, mode, image) =>
+      addUserMessage: (text, mode, image, deviceId) =>
         set((s) => ({
           messages: [
             ...s.messages,
@@ -191,12 +193,13 @@ export const useAppStore = create<AppStore>()(
               content: text,
               mode,
               image,
+              deviceId,
               createdAt: now(),
             },
           ],
         })),
 
-      beginAssistant: (mode) => {
+      beginAssistant: (mode, deviceId) => {
         const id = uid();
         set((s) => ({
           messages: [
@@ -208,6 +211,7 @@ export const useAppStore = create<AppStore>()(
               mode,
               streaming: true,
               tools: [],
+              deviceId,
               createdAt: now(),
             },
           ],