refactor(platform): extract blurActiveElement to infrastructure module

- Centralize duplicated Platform.OS === 'web' blur logic across 16 components
- Add blurActiveElement utility to src/infrastructure/platform module
- Optimize MessageBubble and ImageSegment with React.memo custom comparators
- Add useMemo for computed values in MessageSegmentsRenderer
- Update DTO types with is_system_notice and notice_content fields
- Fix keyboard dismissal order in useChatScreen handleDismiss
- Simplify userStore follow/unfollow state updates
- Remove unnecessary TypeScript type assertions throughout codebase

src/hooks/index.ts

@@ -111,3 +111,16 @@ export type {
   UseDifferentialPostsResult,
   DiffUpdatesInfo,
 } from './useDifferentialPosts';
+
+// ==================== 平台键盘 Hooks ====================
+export {
+  usePlatformKeyboard,
+  useKeyboardAvoidingProps,
+  useKeyboardListener,
+} from './usePlatformKeyboard';
+
+export type {
+  KeyboardEventType,
+  PlatformKeyboardOptions,
+  PlatformKeyboardResult,
+} from './usePlatformKeyboard';