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/components/call/IncomingCallModal.tsx

@@ -13,6 +13,7 @@ import {
 } from 'react-native';
 import { MaterialCommunityIcons } from '@expo/vector-icons';
 import { callStore } from '../../stores/callStore';
+import { blurActiveElement } from '../../infrastructure/platform';
 
 const { height: SCREEN_HEIGHT } = Dimensions.get('window');
 
@@ -26,10 +27,7 @@ const IncomingCallModal: React.FC = () => {
 
   useEffect(() => {
     if (incomingCall) {
-      if (Platform.OS === 'web') {
-        const active = (globalThis as any)?.document?.activeElement as { blur?: () => void } | undefined;
-        active?.blur?.();
-      }
+      blurActiveElement();
       const pulse = Animated.loop(
         Animated.sequence([
           Animated.timing(pulseAnim, {