perf(base-L14): 启动屏背景图移到 public/vent 并修正内联 CSS 引用，修复生产 404

index.html

@@ -55,7 +55,8 @@
           align-items: center;
           flex-direction: column;
           /* background-color: #f4f7f9; */
-          background-image: url('./src/assets/images/vent/loading-bg.png');
+          /* [perf-base-v1] 改引用 public 下静态路径，原 ./src/assets/... 生产构建后 404 */
+          background-image: url('./vent/loading-bg.png');
           background-size: cover;
           background-repeat: no-repeat;
           background-color: var(--vent-base-color);