Change tui dep to ratatui

[dan-da]

Addresses #52

Cargo.toml:

• removes tui dep
• adds ratatui 0.20.0 dep

Also updates dashboard src files to use ratatui instead of tui.

Testing:

Ran neptune-dashboard and verified that all screen/tabs load and appear to look correct.

Notes:

• tui is unmaintained.
• ratatui v0.20.0 appears to be drop-in replacement for tui 0.19.0. It builds and runs without error.
• ratatui v.0.21.0 and above deprecate some things and do not compile cleanly for the dashboard, so minor work will be required should we wish to use a later version. For now though I prefer to keep the PR small so its easier to review.
• I opted to change tui references in code rather than aliasing ratatui to tui in Cargo.toml.