style(ns-newform, diy-form): 对齐后端样式规范优化表单组件

1. 调整ns-newform的图片上传框样式，匹配后端design.css的.img-box样式参数
2. 简化diy-form的样式变量绑定，移除冗余的cssVars计算属性
3. 重构diy-form提交按钮样式，支持宽度百分比和尺寸预设，对齐后端设计规范
4. 优化表单布局类名逻辑，简化样式绑定代码

components/ns-newform/ns-newform.vue

@@ -1302,43 +1302,47 @@ export default {
 				align-items: center;
 				flex-wrap: wrap;
 				gap: 16rpx;
-				
+
+				// 与后端 design.css .form-preview-wrap .img-box 对齐：
+				// 100rpx (≈ 50px) + 1rpx solid #E6E6E6 + 4rpx radius
 				.img-box {
 					margin: 0;
 					display: flex;
 					justify-content: center;
 					align-items: center;
-					width: 120rpx;
-					height: 120rpx;
-					border: 2rpx dashed #d1d5db;
-					border-radius: 8rpx;
+					width: 100rpx;
+					height: 100rpx;
+					border: 1rpx solid #E6E6E6;
+					border-radius: 4rpx;
 					position: relative;
-					background: #f9fafb;
+					background: transparent;
+					box-sizing: border-box;
 					transition: all 0.3s ease;
 					overflow: hidden;
-					
+
 					&:active {
-						background: #f3f4f6;
 						border-color: var(--checked-color, #ff4544);
 					}
-					
+
+					// 关闭按钮：与后端 .form-preview-wrap .img-box .close 对齐
+					// top/right = -14rpx (≈ -7px)，size = 28rpx (≈ 14px)，背景 #999
 					.icon-guanbi {
 						position: absolute;
-						top: -10rpx;
-						right: -10rpx;
+						top: -14rpx;
+						right: -14rpx;
 						display: flex;
 						align-items: center;
 						justify-content: center;
-						width: 40rpx;
-						height: 40rpx;
+						width: 28rpx;
+						height: 28rpx;
 						line-height: 1;
 						color: #fff;
-						background: var(--required-color, #ef4444);
+						background: #999;
 						border-radius: 50%;
-						font-size: 20rpx;
+						font-size: 28rpx;
 						z-index: 10;
 					}
-					
+
 					.icon-add1 {
 						font-size: 48rpx;
 						color: #9ca3af;