fix(reconnect): auto rejoin on socket connect and widen reconnection triggers for mobile foreground resume

- Attempt reconnection on 'disconnected' | 'failed' | 'closed' states (BaseWebRTC)
  - Relax gating: rejoin when roomId exists and any of isPeerDisconnected, isSocketDisconnected, or socketId changed
  - Auto re-join room on socket 'connect' if lastJoinedSocketId differs or not in room; send initiator-online for initiators
  - Track lastJoinedSocketId after successful join and reset isInRoom when socketId changes to bypass early-return
  - Update flows to document mobile background/foreground reconnection and socketId-based rejoin

docs/ai-playbook/flows.zh-CN.md

@@ -634,9 +634,15 @@ async requestWakeLock(): Promise<void> {
 **网络切换适应**：
 
 - **连接检测**：监听 `connectionstatechange` 事件检测网络质量变化
-- **自动重连**：`iceConnectionState: 'disconnected'` 时触发重连流程
+- **自动重连**：`connectionState: 'disconnected' | 'failed' | 'closed'` 时均触发重连流程（统一走 attemptReconnection）
 - **状态恢复**：重连成功后恢复房间状态和传输进度
 
+**移动端后台/前台切换补充策略**：
+
+- **socket 连接恢复自动入房**：`socket.on('connect')` 时，若已持有 `roomId` 且（`lastJoinedSocketId !== socket.id` 或 `!isInRoom`），则强制重新 `joinRoom(roomId, isInitiator, isInitiator)`；发送端会自动广播 `initiator-online`，接收端回复 `recipient-ready`。
+- **身份追踪**：成功 `joinRoom` 后记录 `lastJoinedSocketId = socket.id`，用以检测“后台恢复时 socketId 更换”的情形。
+- **门槛放宽**：`attemptReconnection` 只要满足“`roomId` 存在，且满足任一：P2P 断开 / socket 断开 / socketId 改变”，即可发起重连；不再强依赖“socket 与 P2P 同时断开”。
+
 ### 重连调试要点
 
 **关键日志点**：