Feature request: presenter view of slide mode based on pdfpc metadata

[OrangeX4]

We can implement presenter view like powerpoint based on pdfpc speaker notes in Typst Preview:

• Open two browser pages for two screens, one for presenter and one for audience. The two pages should be synchronized.
• Display pdfpc speaker notes in presenter view (with markdown support).
• Display some helpful information like a timer or the preview of next slide.

These features also require the ability to get the full pdfpc file metadata directly from polylux, just like #191 .