feat: 优先使用 thumbUrl 作为 PDF 封面大图

src/types/post.ts

@@ -30,6 +30,7 @@ export type Attachment = {
   height?: number;
   durationSec?: number;
   posterUrl?: string;
+  thumbUrl?: string;
   thumbnailUrl?: string;
 };
 

src/utils/postResourceAdapter.ts

@@ -26,7 +26,7 @@ function coverFor(att: Attachment | undefined) {
   if (att.kind === "image" || att.mime.startsWith("image/")) {
     return att.thumbnailUrl || att.url;
   }
-  return att.posterUrl || att.thumbnailUrl || "";
+  return att.posterUrl || att.thumbUrl || att.thumbnailUrl || "";
 }
 
 export function postToResource(