Refactored `SenderPdfImageEditor.razor` to improve readability and maintainability by introducing a new wrapper element (`sender-image-editor-shell`) and reorganizing the toolbar into a sticky container (`sender-image-editor__toolbar-host`). Updated conditional rendering logic to handle `PageCount` cases more effectively.
Enhanced styles in `envelope-viewer.css`:
- Added `.sender-image-editor-shell` for layout structure.
- Made toolbar sticky with `.sender-image-editor__toolbar-host`.
- Improved alignment, spacing, and responsiveness of elements.
- Adjusted padding and margins for better layout consistency.
These changes improve user experience and maintainability.