fix: align product detail back button with top mask position

src/pages/product/index.scss

@@ -19,6 +19,9 @@
 }
 
 .back-btn {
+  position: fixed;
+  left: 30rpx;
+  z-index: 41;
   width: 80rpx;
   height: 80rpx;
   flex: 0 0 auto;

src/pages/product/index.tsx

@@ -32,9 +32,14 @@ export default function ProductPage() {
         <View className='product-page'>
           <View className='page-header' style={{ marginTop: `${safe.statusBarHeight + 12}px` }}>
             <View className='page-header-inner'>
-              <View className='back-btn' onTap={() => Taro.navigateBack()}>
+              <View
+                className='back-btn'
+                style={{ position: 'fixed', left: '30rpx', top: `${safe.menuButtonTop}px`, zIndex: 41 }}
+                onTap={() => Taro.navigateBack()}
+              >
                 <Text className='back-arrow'>←</Text>
               </View>
+              <View className='nav-placeholder' />
               <Text className='nav-title'>商品未找到</Text>
               <View className='nav-placeholder' />
             </View>
@@ -79,9 +84,14 @@ export default function ProductPage() {
 
         <View className='page-header' style={{ marginTop: `${safe.statusBarHeight + 12}px` }}>
           <View className='page-header-inner'>
-            <View className='back-btn' onTap={() => Taro.navigateBack()}>
+            <View
+              className='back-btn'
+              style={{ position: 'fixed', left: '30rpx', top: `${safe.menuButtonTop}px`, zIndex: 41 }}
+              onTap={() => Taro.navigateBack()}
+            >
               <Text className='back-arrow'>←</Text>
             </View>
+            <View className='nav-placeholder' />
             <Text className='nav-title'>商品详情</Text>
             <View className='nav-placeholder' />
           </View>