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

package.json

@@ -20,7 +20,7 @@
     "@react-navigation/material-top-tabs": "^7.4.16",
     "@react-navigation/native": "^7.1.31",
     "@react-navigation/native-stack": "^7.14.2",
-    "@shopify/flash-list": "2.0.2",
+    "@shopify/flash-list": "^2.3.1",
     "@tanstack/react-query": "^5.90.21",
     "axios": "^1.13.6",
     "date-fns": "^4.1.0",