Add TUI demo gif to README

[night-slayer18]

The README currently includes a demo gif that showcases the CLI features. To improve documentation and illustrate the TUI capabilities, let's add a similar demonstration gif for the TUI interface as well.

Task:

• Create or record a gif that highlights the TUI (Text-based User Interface) usage and features of leetcode-cli.
• Update the README to include this new gif alongside the existing CLI demo or in the most suitable section.
• Ensure the documentation remains clear and easy to understand for new and existing users.

Additional context:

• The new gif should aim for consistency with the existing demo in presentation style, size, and placement.
• If you need inspiration or a template, refer to the format/positioning of the current demo gif.

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Feel free to suggest placement ideas for the new TUI demo section in the README!