Performance and Optimization, plus Vietnamese url support & readme languages

[Keyyard]

Changelog

[0.5.7] - 2025-05-08

• Refactored all main JavaScript files to use const and let for variable declarations instead of undeclared variables, preventing accidental global scope pollution and improving code clarity.
• Extracted repeated logic into reusable functions:
  • Created utility functions for updating logos, adding download buttons, and removing elements, reducing code duplication and improving maintainability.
• Added element existence checks before accessing properties or methods:
  • Ensured all DOM queries are checked for null/undefined before use, preventing runtime errors if elements are missing.
• Improved error handling:
  • Enhanced error logs with more context for easier debugging.
• General code cleanup:
  • Added 'use strict' to all scripts for safer JavaScript execution.
  • Improved loop variable declarations and used modern JavaScript best practices.
• No functional changes to user-facing features; all improvements are internal for maintainability, reliability, and future-proofing.
• Widen the support urls: Added Vietnamese Studocu.vn url support