Refactor conversation sidebar and enhance UI components for better user experience

- Updated `theme.css` to improve layout and styling of the conversation sidebar, including adjustments to width and padding.
- Enhanced `ConversationSidebar.js` with new elements for improved menu handling and user interactions.
- Streamlined initialization processes in `entrance.js`, `ui.js`, and `ChatView.js` by removing outdated references.
- Updated `index.html` to reflect the new sidebar structure and enhance accessibility features.

Author: GaabDevWeb