Improve color mode preference detection at initialization

[dfsmania]

This pull request refactors and improves the src/ts/color-mode.ts module, enhancing clarity, maintainability, and theme selection logic. The most important changes are grouped below:

Code organization and documentation improvements:

• Improved and standardized section headers and comments throughout the file for better readability, and clarified the file-level documentation.

Theme selection and validation enhancements:

• Introduced a new isValidTheme method to centralize and clarify theme value validation.
• Added the getDataBsTheme method to support reading the theme from the data-bs-theme attribute, improving compatibility with server-side rendering and allowing theme selection via markup.
• Enhanced the theme selection logic in getPreferredTheme to check, in order: localStorage, the markup attribute, and finally OS preference, for a more robust and predictable user experience.

Code clarity and maintainability:

• Improved inline documentation and added comments to clarify logic in event listeners and initialization code, making the code easier to understand and maintain.
• Added a docstring to _prefersDark for consistency.

[netlify]

✅ Deploy Preview for adminlte-v4 ready!

Name	Link
🔨 Latest commit	5f298ad
🔍 Latest deploy log	https://app.netlify.com/projects/adminlte-v4/deploys/6a729ebbd686c3000868872b
😎 Deploy Preview	https://deploy-preview-6093--adminlte-v4.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.