lean-deck v2.1.0

Added

• PDF export — npm run export:pdf -- <deck> prints a whole deck to a vector PDF (one slide per 1920×1080 page)
  through the new ?print=1 viewer mode and the Chrome DevTools protocol. No screenshots, no extra dependencies;
  it fails when the page count does not match the slide count.

Changed

• The background grid is drawn as line paths instead of a CSS gradient. It looks the same on screen
  and no longer turns into coarse tiles in PDF viewers such as Preview.