bugfix: 没有产品主图时自动选中问题

src/views/SellerDashboard/MyListings/EditDetail/agents.md

@@ -50,6 +50,7 @@ When saving, preserve the backend's expected image categories. Confirm backend n
 
 - The `main` badge represents the first selected product image, not the most recently selected one.
 - `firstSelectedIndex` is stored on each `ListingItem` and passed to `ProductImageList.vue`.
+- Hydrating detail data must only set `mainProductImage` from explicit `main_product` or `mainProductImage` rows. Never infer it from selected `product` rows.
 - Selecting a product image should only set `mainProductImage` when no main image is currently tracked by that listing's `firstSelectedIndex`.
 - Unselecting the current main product image clears `mainProductImage` and resets `firstSelectedIndex`.
 - Videos can be selected and saved, but they cannot become `mainProductImage`, must not set `firstSelectedIndex`, and must not display the `main` badge.
@@ -57,6 +58,7 @@ When saving, preserve the backend's expected image categories. Confirm backend n
 中文补充：
 
 - `main` 标识只给图片，不给视频。
+- 数据回显时，只有接口明确返回 `main_product` / `mainProductImage` 才能设置主图。不要因为某个 `product` 是已选中状态，就自动把它当成 `mainProductImage` 或显示 `main`。
 - 第一次选择视频时可以弹 warning，但视频本身仍然要保持选中；只是不要把它写入 `mainProductImage`，也不要更新 `firstSelectedIndex`。
 - 如果先选中视频，再选中图片，图片仍然可以成为第一个主图。
 - 如果取消的是当前主图图片，需要清空 `mainProductImage` 和 `firstSelectedIndex`；取消普通图片或视频不应影响主图。

src/views/SellerDashboard/MyListings/EditDetail/index.vue

@@ -313,10 +313,6 @@
 				if (videoItem) listing.prodImageList.push(videoItem)
 			})
 
-		if (!listing.mainProductImage) {
-			listing.mainProductImage =
-				listing.prodImageList.find((item) => item.selected && !item.isVideo)?.url || ""
-		}
 		if (coverFromImageUrl) {
 			listing.coverFrom = resolveCoverSourceFromImageUrl(coverFromImageUrl, listing)
 		}
@@ -324,11 +320,7 @@
 		const mainProductIndex = listing.prodImageList.findIndex(
 			(item) => !item.isVideo && item.url === listing.mainProductImage
 		)
-		const selectedIndex =
-			mainProductIndex === -1
-				? listing.prodImageList.findIndex((item) => item.selected && !item.isVideo)
-				: mainProductIndex
-		listing.firstSelectedIndex = selectedIndex === -1 ? null : selectedIndex
+		listing.firstSelectedIndex = mainProductIndex === -1 ? null : mainProductIndex
 
 		listing.productImage = listing.prodImageList.map((item) => item.url)
 		listing.apparelSketch = listing.sketchList