fix: unify product detail back button with top mask style

src/pages/shop/detail/index.scss

@@ -281,10 +281,10 @@
 .shop-back-btn {
   position: fixed;
   top: calc(env(safe-area-inset-top, 20px) + 20rpx);
-  left: 24rpx;
+  left: 30rpx;
   z-index: 50;
-  width: 88rpx;
-  height: 88rpx;
+  width: 80rpx;
+  height: 80rpx;
   border-radius: 50%;
   background: rgba(40, 35, 32, 0.5);
   backdrop-filter: blur(16px) saturate(1.4);
@@ -302,9 +302,10 @@
 }
 
 .shop-back-arrow {
-  font-size: 36rpx;
-  color: var(--text-primary);
+  font-size: 44rpx;
+  font-weight: 600;
   line-height: 1;
+  color: var(--text-primary);
 }
 
 /* --- Bottom Action Bar — 毛玻璃由 BottomActionBar 提供，本地只补充价格摘要 --- */

src/pages/shop/detail/index.tsx

@@ -277,7 +277,7 @@ export default function ShopDetailPage() {
       <View
         className="shop-back-btn"
         onTap={() => Taro.navigateBack()}
-        style={{ top: `${safe.backBtnPaddingTop}px`, opacity: heroStyles.backBtnOpacity }}
+        style={{ top: `${safe.menuButtonTop}px`, opacity: heroStyles.backBtnOpacity }}
       >
         <Text className="shop-back-arrow">←</Text>
       </View>