Accept PDF in Croptop drag-n-drop

[livid]

• [x] Preview in the Quick Share dialog
• [x] Generate _preview.png from the first page and use it as og:image

[livid]

Drop to the app icon works.

[livid]

• [x] Drop to the app content area
• [x] Drop to the quick share sheet

[livid]

If PDF is the only asset in the post, we should generate a preview, like with video.