Prep everything for Chrome Web Store submission

To publish the extension so anyone can "Add to Chrome," Google requires
a few things people get stuck on - prepared them all:

- /privacy page (templates/privacy.html + route + site-wide footer
  link): the store requires a privacy-policy URL for an extension with
  host + scripting permissions. States plainly that it reads only the
  user's own public pages, collects nothing, sends nothing to the
  server.
- Demo mode on /progress (?demo=1): renders a few sample draft cards so
  people can see the output without running the bookmarklet - also used
  to generate the store screenshots, and handy as a "see an example"
  link.
- extension/store-assets/*.png: two 1280x800 screenshots (results +
  landing) meeting the store's screenshot requirement.
- extension/STORE_SUBMISSION.md: paste-ready listing copy (name,
  summary, description, category), permission justifications, privacy
  answers, and the step-by-step. The $5 dev account + upload is the
  user's to do.
- extension.zip is generated and gitignored (build artifact).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>