refactor: streamline post sync, fix image gallery, and clean up chat screen

- **Post sync optimization**: Clear store immediately when params change to prevent flashing old posts; replace instead of merge on refresh
- **ImageGallery fix**: Use idempotent download option, migrate to Asset.create() API, fix Android file path requirement, ensure proper cleanup
- **UserProfileScreen**: Add ImageGallery for post image viewing with consistent implementation
- **SearchScreen**: Add entrance animation and empty state
- **ChatScreen**: Remove unused state variables (lastSeq, firstSeq, isProgrammaticScrollRef) and dead imports

src/components/common/ImageGallery.tsx

@@ -213,6 +213,7 @@ export const ImageGallery: React.FC<ImageGalleryProps> = ({
     }
 
     setSaving(true);
+    let downloaded: InstanceType<typeof File> | null = null;
     try {
       const urlPath = currentImage.url.split('?')[0];
       const ext = urlPath.split('.').pop()?.toLowerCase() ?? 'jpg';
@@ -222,13 +223,14 @@ export const ImageGallery: React.FC<ImageGalleryProps> = ({
       const fileName = `withyou_${Date.now()}.${fileExt}`;
       const destination = new File(Paths.cache, fileName);
 
-      // File.downloadFileAsync 是新版 expo-file-system/next 的静态方法
-      const downloaded = await File.downloadFileAsync(currentImage.url, destination);
+      // File.downloadFileAsync 是新版 expo-file-system 的静态方法
+      downloaded = await File.downloadFileAsync(currentImage.url, destination, {
+        idempotent: true,
+      });
 
-      await MediaLibrary.saveToLibraryAsync(downloaded.uri);
-
-      // 清理缓存文件
-      downloaded.delete();
+      // expo-media-library v17+ 已移除顶层 saveToLibraryAsync，改用 Asset.create()
+      // 在 Android 上 filePath 必须以 file:/// 开头
+      await MediaLibrary.Asset.create(downloaded.uri);
 
       onSave?.(currentImage.url);
       showToast('success');
@@ -236,6 +238,14 @@ export const ImageGallery: React.FC<ImageGalleryProps> = ({
       console.error('[ImageGallery] 保存图片失败:', err);
       showToast('error');
     } finally {
+      // 清理缓存文件（无论成功失败都清理，避免残留）
+      if (downloaded) {
+        try {
+          downloaded.delete();
+        } catch (cleanupErr) {
+          console.warn('[ImageGallery] 清理缓存文件失败:', cleanupErr);
+        }
+      }
       setSaving(false);
     }
   }, [currentImage, saving, onSave, showToast]);