Updated `envelope-viewer.css` to enhance the layout and responsiveness of the PDF toolbar:
- Adjusted padding, gap, width, and alignment for better usability.
- Improved zoom section and slider styles for flexibility and consistency.
Enhanced `pdf-viewer.js` to handle concurrent rendering tasks:
- Added checks to prevent overlapping render tasks on the same canvas.
- Implemented error handling for rendering operations to ensure stability.
These changes improve the user experience and robustness of the PDF viewer.