Add support for importing and displaying PDF files in Nextcloud Whiteboard

Currently the Nextcloud Whiteboard app only allows images to be added to the canvas. For collaborative brainstorming workflows, it is essential to be able to import PDF documents directly, display them on the canvas, and annotate them in place.

Use case:
When working with teams on design reviews, architectural concepts or shared documents (such as meeting minutes, sketches, plans), PDFs are one of the most common formats. Users often want to bring existing PDF content (e.g. plans, diagrams, reports) into the whiteboard, annotate them together, and keep related annotations contextually integrated on the same canvas.

Proposed functionality:
1. Support uploading PDF files directly into the Whiteboard.
2. Render PDF pages so that they can be displayed inline (similar to images).
3. Allow users to navigate pages of a PDF within the board (e.g. by selecting page numbers).
4. Allow basic whiteboard interactions/annotations on top of the rendered PDF content.
5. Bonus: Option to export annotated PDF back to PDF with annotations embedded.

Benefits:
• Enables richer collaborative workflows without switching between apps.
• Reduces need for external screenshot-to-image conversion workarounds.
• Improves adoption of Whiteboard for workshops, design reviews and teaching.

The issue does not name files or tests. Start by locating the existing image import and display flow in the JavaScript Whiteboard code, then assess the requested PDF upload, page navigation, inline rendering, and annotation behavior. Done means PDFs can be added, browsed, displayed, and annotated on the canvas; exporting annotated PDFs is listed as a bonus.