style(GradeClassSelector): 更新表单标签样式并添加必填项标记

添加必填项的红色星号标记，并调整标签字体粗细以提升可读性

src/components/common/GradeClassSelector.vue

@@ -164,11 +164,15 @@ export default {
   flex: 1;
 }
 
+.form-item.required .form-label::after {
+  content: " *";
+  color: #ff4d4f;
+}
+
 .form-label {
   display: block;
   margin-bottom: 8px;
-  font-size: 14px;
-  color: #333;
+  font-weight: 500;
 }
 
 .grade-selector,