fix: Replace outdated validate-color third party library

[aed2fe]

Pull Request Template

Description

On my Gnome-Shell (Version 46.0) from Ubuntu 24.04 I got the error:

SyntaxError: ambiguous indirect export: validateHTMLColorHex @ file:///home/aed2fe/.local/share/gnome-shell/extensions/pano@elhan.io/extension.js:59:9

After looking into it I found out that the library validate-color is outdated since 2 years. So i decided to remove the library and implement the feature covered by it as sub-module here.

HINT: As I'm not an experienced TypeScript developer I used the help of the GitHub Copilot with the LLM Claude Sonnet 4.5

Fixes # (issue)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the style guidelines of this project
• My commits follow the commit standards of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have checked my code and corrected any misspellings