refactor(PostCard, ImageGrid, SmartImage): enhance post editing display and image handling

- Introduced a new method to determine the last edited time for posts, prioritizing content_edited_at for better accuracy in display.
- Updated PostCard and PostDetailScreen components to utilize the new editing display logic.
- Enhanced ImageGrid and SmartImage components to support additional preview URLs and improve image loading behavior, including lazy loading optimizations for web.
- Modified Post and PostDTO interfaces to include contentEditedAt for better tracking of post edits.

src/types/dto.ts

@@ -74,6 +74,8 @@ export interface PostDTO {
   is_vote: boolean;
   created_at: string;
   updated_at?: string;
+  /** 用户编辑正文/标题/图片的时间；优先用于「已编辑」展示 */
+  content_edited_at?: string;
   author: UserDTO | null;
   is_liked: boolean;
   is_favorited: boolean;