Support LinkedIn Document / Carousel Posts (PDF Upload)

What problem does this solve?
LinkedIn carousel (document) posts generate significantly higher engagement and dwell time than standard image or text posts. Currently, Mixpost does not support uploading and scheduling PDF documents as native LinkedIn carousels, forcing users to post them manually via the native LinkedIn interface.

Describe the solution
Add support for PDF document uploads in the post composer specifically for LinkedIn accounts. When a PDF is attached, Mixpost should publish it as a swipeable multi-page carousel with an optional document title.

Alternatives considered
Manually scheduling/publishing document posts directly on LinkedIn.