feat(video): 事务化切换与前端统一编排，增强视频输入格式支持

- 后端：切换事务+transition_id，/stream/mode 返回 switching/transition_id 与实际 codec

- 事件：新增 mode_switching/mode_ready，config/webrtc_ready/mode_changed 关联事务

- 编码/格式：扩展 NV21/NV16/NV24/RGB/BGR 输入与转换链路，RKMPP direct input 优化

- 前端：useVideoSession 统一切换，失败回退真实切回 MJPEG，菜单格式同步修复

- 清理：useVideoStream 降级为 MJPEG-only

web/src/api/index.ts

@@ -227,10 +227,10 @@ export const streamApi = {
   getSnapshotUrl: () => `${API_BASE}/snapshot`,
 
   getMode: () =>
-    request<{ success: boolean; mode: string; message?: string }>('/stream/mode'),
+    request<{ success: boolean; mode: string; transition_id?: string; switching?: boolean; message?: string }>('/stream/mode'),
 
   setMode: (mode: string) =>
-    request<{ success: boolean; mode: string; message?: string }>('/stream/mode', {
+    request<{ success: boolean; mode: string; transition_id?: string; switching?: boolean; message?: string }>('/stream/mode', {
       method: 'POST',
       body: JSON.stringify({ mode }),
     }),