Summary

Adds WithFooter(string) and WithFooterColor(Color) to ModalNode, enabling text to be rendered in the bottom
border line — mirroring the existing WithTitle/WithTitleColor pattern for the top border.

Use case: Keyboard hints, status text, or context info displayed directly in the modal frame without consuming
content area space.

var modal = new ModalNode()
    .WithTitle(" Settings ")
    .WithTitleColor(Color.BrightCyan)
    .WithFooter(" Esc: Close | Enter: Save ")
    .WithFooterColor(Color.Gray);

Rendering:
╭─ Settings ────────────────────────────╮
│ content...                            │
╰─ Esc: Close | Enter: Save ────────────╯

Changes

• ModalNode.cs: Added _footer/_footerColor fields, WithFooter()/WithFooterColor() fluent methods, and footer rendering in RenderPanel() at the bottom border line (position 2, y = bounds.Height - 1)
• Footer text is truncated to bounds.Width - 4 if too long
• Custom color applied if set, otherwise uses default terminal colors
• Zero breaking changes — fully additive, footer is null by default

Fixes #

Changes

Please provide a brief description of the changes here.

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.
• Design discussion issue #
• Changes in public API reviewed, if any.
• I have added website documentation for this feature.

Latest dev Benchmarks

Include data from the relevant benchmark prior to this change here.

This PR's Benchmarks

Include data from after this change here.