style(board): 更新安灯看板背景样式

- 移除背景图片设置
- 添加纯色背景 #0f2740
- 保持容器尺寸和定位属性不变

src/views/board/andonBoard/index.vue

@@ -454,9 +454,7 @@ export default {
 
 <style scoped lang="less">
 .app-container {
-  background-image: url("../../../assets/board/boardBg2.jpg");
-  background-repeat: no-repeat;
-  background-size: 100% 100%;
+  background: #0f2740;
   width: 100%;
   height: 100%;
   position: absolute;