v2.0.70: Code organization improvements

🔧
Fix : Description was not replaced when navitating to new videos through SPA.

Improvements

• Centralize observers in dedicated file
• Update description log color for better visibility
• Update extension description text

📝 Details

1. Move all DOM observers to observer.ts for better code organization
2. Change description logs color to blue for better readability
3. Update extension's description text for better clarity
4. Apply single responsibility principle by isolating observer logic