Major code changes.

[arabcoders]

This pull request includes several changes aimed at improving the functionality and consistency of the codebase, particularly focusing on the download management system and API endpoints. The most important changes include adding new API endpoints, renaming variables for consistency, and enhancing the DownloadQueue class.

API Enhancements:

• Added new API endpoints: POST /api/history for adding multiple links and GET /api/history/add for adding a single item via GET request.

Consistency Improvements:

• Renamed canceled to cancelled across multiple files to maintain consistency in variable naming. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
• Renamed output_template to template and output_template_chapter to template_chapter for clarity and consistency. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

DownloadQueue Class Enhancements:

• Converted DownloadQueue to a singleton class to ensure a single instance manages the download queue and history.
• Added detailed docstrings to methods in DownloadQueue to improve code readability and maintainability.

Configuration and Settings:

• Updated .vscode/settings.json to include new settings for CSS style sheets and spell checking in various document types.