feat: Add image error handling and improve styles

- Added a new `addErrorHandler` method in `html.mjs` to handle missing images by providing a fallback placeholder and applying CSS classes for error styling. This ensures a smoother user experience when images fail to load.
- Updated the `getTemplate` method in `html.mjs` to utilize the new `addErrorHandler` method, thereby enhancing functionality with error management integrated directly in the HTML assembly.
- Refactored the `initEventHandler` method in `message.mjs` to directly set content options without unnecessary delays, which improves content rendering and responsiveness.
- Enhanced CSS transitions in `html.pcss` and `message.pcss`, ensuring images and divs smoothly transition, which enhances visual appeal during UI updates.
- Changed a background color in `html.pcss` to provide better contrast and visual clarity.