Accessibility: add colorblind-friendly themes for diff views

Please add colorblind-friendly themes to GitHub Copilot Desktop, similar to the GitHub Colorblind themes in VS Code.

This matters most in diff views, where additions and deletions rely on red and green. Those colors are difficult to distinguish for people with red-green color-vision deficiency, making change review harder.

A colorblind-friendly diff palette or non-color cues for additions and deletions would make the review experience more accessible.

No implementation files or tests are named. Start by comparing the GitHub Colorblind themes in VS Code with the Copilot Desktop diff views, then identify how additions and deletions are currently presented. Done means the diff experience offers a colorblind-friendly palette or non-color cues that make both change types distinguishable.

索引モデルが issue の本文から書いたものです。