Fix input under iOS keyboard

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Description

This PR fixes the issue with the jumping keyboard on iOS devices. It implements the useVisualViewport hook to calculate the height of the real viewport and sets the height of the message container so that the message input is positioned below the keyboard.

Summary of Changes

Created a new file useVisualViewport.js in the src/hooks directory to implement the useVisualViewport hook.
Modified App.jsx in the src/components/App directory to import and use the useVisualViewport hook.
Added a new state variable viewportHeight to hold the height of the viewport.
Used the useVisualViewport hook to get the height of the viewport and update the state variable.
Set the height of the message container using the viewportHeight state variable.

Please review and merge these changes to resolve the issue with the iOS keyboard.

Fixes #12.

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