fix: 移动端样式优化 (#1979)

#### What this PR does / why we need it?

#### Summary of your change

#### Please indicate you've done the following:

- [ ] Made sure tests are passing and test coverage is added if needed.
- [ ] Made sure commit message follow the rule of [Conventional Commits specification](https://www.conventionalcommits.org/).
- [ ] Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

frontend/src/layout/index.vue

@@ -148,7 +148,6 @@ onMounted(() => {
     top: 0;
     bottom: 0;
     left: 0;
-    z-index: 9999;
     overflow: hidden;
 }
 
@@ -178,6 +177,7 @@ onMounted(() => {
         transition: transform 0.28s;
         width: var(--panel-menu-width) !important;
         background: #ffffff;
+        z-index: 9999;
     }
     .app-footer {
         display: block;