custom-tab-bar: 调整 + 号按钮居中样式，使用 align-self: center;

pages/template/custom-tab-bar/custom-tab-bar.uvue

@@ -179,14 +179,13 @@
 
   .btn-plus {
     position: absolute;
-    left: 50%;
     width: 70px;
     height: 70px;
     bottom: 20px;
     border-radius: 50px;
     background-color: #FE5722;
     box-shadow: 0 0 4px rgba(0, 0, 0, 0.5);
-    margin-left: -35px;
+    align-self: center;
     align-items: center;
     justify-content: center;
     overflow: visible;