Point all URLs at the real service URL (reflip-7xac.onrender.com)

The new Render service got a suffixed URL (reflip-7xac.onrender.com)
since bare 'reflip' was taken. Synced every hardcoded reference to it -
extension scripts, background.js HOMEPAGE, OG image (regenerated),
README, and the store submission doc. Confirmed the new service serves
the app (home + /privacy both 200). extension.zip rebuilt.

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