feat: add vertical scrolling mode for PDF viewer

[cameronaaron]

feat: add vertical scrolling mode for PDF viewer

• Implemented vertical scrolling mode to display all PDF pages in a continuous scroll view.
• Added styles for vertical layout and page number indicators in CSS.
• Enhanced JavaScript logic for rendering all pages and tracking current page based on scroll position.
• Introduced a toggle button in the toolbar for switching between vertical scroll mode and traditional page mode.
• Updated Android integration to support vertical scroll mode and preserve state during mode switching.
• Added new vector drawable for the vertical scroll toggle icon.