refactor: add element existence checks and improve chatbot initializa…

[Natwar589]

…tion robustness

• Added ensureInterfaceElements method to verify and recreate missing DOM elements before opening chatbot
• Added ensureStylesheetInjected method to ensure stylesheet is loaded when needed
• Updated initializeChatbot to check document.readyState before adding DOMContentLoaded listener
• Wrapped loadContent logic in try-catch block with error logging
• Added document.body availability check in loadContent before