Plugin slide decks get full deck navigation and export

Slides whose type comes from a plugin — such as Publish/Slide — previously rendered without their deck context: the counter stuck at "Slide 1 / 1", Prev/Next never appeared, and a plugin-typed deck offered no Export menu and no PDF export. The platform now recognizes plugin slide and deck types everywhere the built-in Slide and Deck types are recognized: deck navigation, the presenter bar, the Export menu, pixel-faithful PDF export, and HTML export.