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/services/post/PostSyncService.ts

@@ -62,23 +62,36 @@ class PostSyncService {
   }
 
   async fetchPosts(params?: GetPostsParams, key: string = 'default'): Promise<PostsResult> {
-    const store = usePostListStore.getState();
-    store.updatePostsState(key, { isLoading: true, error: null });
+    const queryParams: GetPostsParams = {
+      pageSize: params?.pageSize || DEFAULT_PAGE_SIZE,
+      ...params,
+      cursor: params?.cursor !== undefined ? params.cursor : '',
+    };
+
+    // 判断参数是否变化：参数变化（切换分区/排序/筛选）时直接清空旧数据，
+    // 避免渲染期间残留上一个分区的帖子造成闪烁
+    const prevState = usePostListStore.getState().getPostsState(key);
+    const paramsChanged = JSON.stringify(prevState.lastParams) !== JSON.stringify(queryParams);
+
+    // 参数变化时立即写入空数据 + loading 状态，确保 UI 第一时间进入加载态
+    // 而不是先渲染上一个分区的旧帖子
+    usePostListStore.getState().updatePostsState(key, {
+      isLoading: true,
+      error: null,
+      ...(paramsChanged ? { posts: [], cursor: null, currentPage: 1, hasMore: true } : {}),
+    });
 
     try {
-      const queryParams: GetPostsParams = {
-        pageSize: params?.pageSize || DEFAULT_PAGE_SIZE,
-        ...params,
-        cursor: params?.cursor !== undefined ? params.cursor : '',
-      };
-
       const result = await postRepository.getPosts(queryParams);
       const isCursorMode = result.nextCursor !== undefined && result.nextCursor !== null;
 
       const currentState = usePostListStore.getState().getPostsState(key);
       const incomingPosts = Array.isArray(result.posts) ? result.posts : [];
       const mergeStart = Date.now();
-      const mergedPosts = mergeRefreshWindow(currentState.posts, incomingPosts);
+      // 参数变化时直接替换；同参数刷新则合并以保留滚动位置上的帖子
+      const mergedPosts = paramsChanged
+        ? incomingPosts
+        : mergeRefreshWindow(currentState.posts, incomingPosts);
       const mergeCost = Date.now() - mergeStart;
       if (mergeCost >= MERGE_PERF_WARN_THRESHOLD_MS) {
         console.log('[PostPerf] fetchPosts merge cost', { key, costMs: mergeCost, prev: currentState.posts.length, next: incomingPosts.length });
@@ -116,7 +129,8 @@ class PostSyncService {
       const isCursorMode = result.nextCursor !== undefined && result.nextCursor !== null;
       const incomingPosts = Array.isArray(result.posts) ? result.posts : [];
       const mergeStart = Date.now();
-      const mergedPosts = mergeRefreshWindow(state.posts, incomingPosts);
+      // refreshPosts 是刷新操作，应直接替换当前列表，避免旧数据残留
+      const mergedPosts = incomingPosts;
       const mergeCost = Date.now() - mergeStart;
       if (mergeCost >= MERGE_PERF_WARN_THRESHOLD_MS) {
         console.log('[PostPerf] refreshPosts merge cost', { key, costMs: mergeCost, prev: state.posts.length, next: incomingPosts.length });