fix(mentions): render actual user nicknames in @mention display

- Add displayName variable using atData.nickname with fallback to generic text
- Replace hardcoded '@所有人 ' and '@某人 ' with dynamic '@{displayName} ' format

src/components/business/PostContentRenderer.tsx

@@ -61,6 +61,7 @@ const PostContentRenderer: React.FC<PostContentRendererProps> = ({
         const atData = segment.data as AtSegmentData;
         const userId = atData?.user_id;
         const isAtAll = userId === 'all';
+        const displayName = atData?.nickname || (isAtAll ? '所有人' : '某人');
         elements.push(
           <Text
             key={key}
@@ -72,7 +73,7 @@ const PostContentRenderer: React.FC<PostContentRendererProps> = ({
               },
             ]}
           >
-            {isAtAll ? '@所有人 ' : '@某人 '}
+            @{displayName}{' '}
           </Text>
         );
         break;