Seamless Offline Mode with Local Message Caching in Flutter

Introduce support for offline mode and local message caching to ensure a smooth and reliable chat experience. Users should be able to instantly view previously loaded messages when opening a conversation, regardless of network connectivity. When the device is online, fresh messages should be fetched and intelligently merged with or replace cached data. Storage usage must be automatically managed and kept within limits, and the UI should gracefully handle offline mode without errors or interruptions. This feature will significantly improve usability during poor or unavailable network conditions while maintaining accuracy once reconnected.