feat(performance): migrate FlatList to FlashList and add animations

Replace FlatList with FlashList across all message screens (ChatScreen, MessageListScreen, NotificationsScreen, HomeScreen) for improved list virtualization performance. Use `drawDistance={250}` instead of manual pagination. Simplify React.memo comparisons in MessageBubble and SegmentRenderer by removing function prop checks to prevent unnecessary re-renders.

Add AsyncStorage persistence for lastSystemMessageAt to avoid showing current time on first render. Include enter animations (fade and slide) for CreateGroupScreen and modernize UI styling to flat design.

BREAKING CHANGE: Upgrade @shopify/flash-list from 2.0.2 to ^2.3.1

src/stores/message/index.ts

@@ -17,6 +17,7 @@ export {
   useMessageStore,
   normalizeConversationId,
   mergeMessagesById,
+  loadPersistedLastSystemMessageAt,
 } from './store';
 export type { MessageState, MessageActions, MessageStore } from './store';