Add 'Add to Chrome' + 'See an example' CTAs to the landing hero

- Primary "Add to Chrome" button, driven by a CHROME_STORE_URL env
  var: unset (as now, pre-approval) it renders a disabled "Add to
  Chrome — soon" chip so there's no broken link; set it once the
  extension is approved and it flips to a live store link with no code
  change (just a Render env var + restart).
- Secondary "See an example" button linking to the ?demo=1 results
  page, so visitors who aren't ready to install can see the actual
  output first.

Verified both states render (disabled default, live link when the env
var is set) and the example link points at the demo page.

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