feat: "Need a laptop?" ad on prompt + os pages

- prompt.mjs: button text changed from "laptop" to "Need a laptop?"
- os.mjs: added "Need a laptop?" section with a "blank" button that
  jumps to the blank piece

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: whistlegraph