Updated `.pdf-viewer-container` to vertically center content by changing `align-items` to `center`. Adjusted `.pdf-frame` dimensions to use dynamic height and width, added a maximum width, and centered text alignment. Introduced rounded corners to the gradient bar with a `border-radius`. Changed `.pdf-canvas` to `inline-block` display with `vertical-align: top`. Further refined `.pdf-frame` layout with slightly different height and width calculations.
Moved inline styles from `EnvelopeViewer.razor` to a new external
CSS file `envelope-viewer.css` for better maintainability and
separation of concerns. Added external JavaScript dependencies
(`pdf.js` and `pdf-viewer.js`) to enable PDF rendering.
Fixed encoding issues in error messages and improved the layout
structure with responsive design rules. Enhanced readability and
usability by organizing styles and ensuring proper localization
support.
