Rebrand URL to crosslister.onrender.com (direction-neutral)

The old subdomain (depop-to-vinted-web) implied one direction; the tool
goes both ways. Renamed to "Crosslister" - brandable, direction-neutral,
and matches the extension's name.

Synced every hardcoded reference to crosslister.onrender.com: the
extension's bundled scripts (depop.js, vinted.js, background.js), the
OG share image (regenerated - it renders the URL as text), the store
submission doc, and the README. The app's own URLs are dynamic
(request.url_root / url_for _external) so the site auto-adapts to
whatever domain serves it - no app.py change needed. extension.zip
rebuilt.

Requires the Render service to be renamed to "crosslister" in the
dashboard (Settings -> Name) for the new URL to go live.

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