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/screens/home/HomeScreen.tsx

@@ -437,7 +437,6 @@ export const HomeScreen: React.FC = () => {
     refreshing: isRefreshing,
     hasMore,
     error,
-    reset,
   } = useDifferentialPosts<Post>(
     [],
     {
@@ -497,8 +496,9 @@ export const HomeScreen: React.FC = () => {
   useEffect(() => {
     let cancelled = false;
     const run = async () => {
-      reset();
-      await new Promise(resolve => requestAnimationFrame(resolve));
+      // 注意：不再在此处调用 reset()。
+      // fetchPosts 内部会在参数变化时立即清空 store 中的旧数据并设置 loading，
+      // 这样可以避免本地清空后、请求发起前的间隙渲染到上一个分区的帖子造成闪烁。
       if (!cancelled) {
         await refresh();
       }