fixed ascending/descending sorting order toggle function and formatted code

[YianXie]

Summary

This pull request focuses on implementing the ascending/descending sorting order toggle function and formatting code.**

In terms of sorting order:

Modified index.js so that it automatically changed the sorting order based on the user's interaction with the tab at the top. The direction of the arrow is also automatically changed based on the sorting order.

In terms of code formatting (summary written by copilot):

This pull request primarily refactors code for consistency and readability across backend PHP files, frontend HTML files, and CSS stylesheets. The most significant changes include reformatting PHP function definitions and control structures to use standard indentation and brace style, improving HTML formatting for better readability and maintainability, and making minor adjustments to CSS for consistency.

Backend PHP code formatting improvements:

• Reformatted all PHP function definitions and control structures in backend/data.php and backend/problems_importer.php to use consistent indentation and brace placement, enhancing readability and maintainability. [1] [2]

Frontend HTML formatting and accessibility:

• Updated frontend/about.html and frontend/faq.html to use consistent indentation, self-closing tags, and multi-line attributes for improved readability and easier future modifications. [1] [2]
• Improved navigation bar and dropdown formatting in HTML files for accessibility and clarity. [1] [2] [3]

CSS consistency fixes:

• Standardized color values and indentation in css/main.css and css/utility.css for consistent styling. [1] [2]

Documentation:

• Added a blank line after the main heading in README.md for improved markdown readability.