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.

未指定任何实现文件或测试。首先比较 VS Code 中的 GitHub Colorblind 主题与 Copilot Desktop 的 diff 视图，然后确定当前如何呈现添加和删除内容。完成标准是 diff 体验提供适合色盲用户的配色方案，或提供不依赖颜色的提示，使两种更改类型都能被区分。

由索引模型根据 Issue 内容生成。