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/hooks/useDifferentialPosts.ts

@@ -236,6 +236,14 @@ export function useDifferentialPosts<T extends PostIdentifier = Post>(
     const currentState = usePostListStore.getState().getPostsState(listKey);
     syncFromStore(currentState);
 
+    // listKey 切换时，如果目标列表在 store 中还没有有效数据（首次访问），
+    // 立即进入 loading 态，避免先渲染空列表再切到 loading 造成闪烁。
+    // syncFromStore 会用 store 的 isLoading 覆盖，因此在其之后补设。
+    // 如果已有数据（曾访问过），则保留旧数据，由后续 fetch 静默刷新。
+    if (!currentState.posts || currentState.posts.length === 0) {
+      setLoading(true);
+    }
+
     const unsubscribe = usePostListStore.subscribe(state => {
       const postsState = state.postsStateMap.get(listKey);
       if (postsState) syncFromStore(postsState);
@@ -277,6 +285,18 @@ export function useDifferentialPosts<T extends PostIdentifier = Post>(
   const reset = useCallback(() => {
     calculatorRef.current?.reset();
     batcherRef.current?.clearPending();
+    // 同步清空 store 中对应 listKey 的数据，避免 subscribe 回调
+    // 在切换 listKey 时把上一个 key 的残留数据重新同步回本地造成闪烁
+    usePostListStore.getState().updatePostsState(listKey, {
+      posts: [],
+      cursor: null,
+      currentPage: 1,
+      hasMore: true,
+      isLoading: false,
+      isRefreshing: false,
+      error: null,
+      lastParams: undefined,
+    });
     setPosts([]);
     setLoading(false);
     setRefreshing(false);
@@ -284,7 +304,7 @@ export function useDifferentialPosts<T extends PostIdentifier = Post>(
     setHasMore(true);
     previousPostsRef.current = [];
     setDiffUpdates({ addedCount: 0, updatedCount: 0, deletedCount: 0, lastUpdateTime: 0 });
-  }, []);
+  }, [listKey]);
 
   const forceUpdate = useCallback((newPosts: T[]) => {
     setPosts(newPosts);