Performance Improvements:
- Changed offline_messages from Vec to VecDeque for efficient FIFO operations
- Replaced pop() with pop_front() for proper queue behavior in message processing
- Used push_back() instead of push() for correct message ordering in storage

Structural Changes:
- Added explicit VecDeque import for clarity
- Updated OfflineInfo struct to use VecDeque for offline_messages
- Maintained all existing functionality with improved performance

Key Benefits:
1. **Efficiency**: VecDeque provides O(1) amortized time for both ends
2. **Correctness**: Ensures proper first-in-first-out delivery of offline messages
3. **Memory**: Better memory utilization for queue operations
4. **Ordering**: Maintains message sequence during offline storage and retrieval

The changes ensure that offline messages are processed in the correct order while improving performance for queue operations, maintaining full backward compatibility.