fix: center message input box on large screens
Moved alignItems: 'center' from inputContainerCentered to base inputContainer
style to ensure the input box (with maxWidth: 700px) is always centered,
matching the message list centering.
Changes:
- inputContainer: Added alignItems: 'center' for consistent centering
- inputContainerCentered: Removed alignItems (only keeps justifyContent for empty state)