docs: refresh architecture overview

[vultuk]

This pull request updates the CONTRIBUTING.md documentation to clarify terminology and improve the project structure overview. The most important changes are grouped below:

Documentation improvements:

• Corrected spelling from "behavior" to "behaviour" for consistency in issue reporting guidelines.

Project structure clarification:

• Updated the markdown-web/ directory structure to include new server-side files: api.ts, auth.ts, https-server.ts, and themeManager.ts, and clarified their purposes in the documentation.

[Copilot]

Pull Request Overview

This PR refreshes the project's documentation by updating the architecture overview and standardising language to UK English. The changes improve clarity about the server structure and ensure consistent British English usage throughout the contributing guidelines.

• Documents complete server file structure including authentication, HTTPS, and theme management components
• Standardises CONTRIBUTING.md language from American to British English spelling

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}