Improve error handling, refactor code, and enhance maintainability

- Added error handling to TRANSLATION_API_CALL to manage fetch errors.
- Ensured observer is initialized before disconnection in content.js.
- Refactored translation logic into a separate function to avoid duplication.
- Changed let to const for non-reassigned variables.
- Converted .then() to await in popup.js for async/await consistency.

code refactoring, fixed minor bug, added new gitignore file