_join_url_parts() only checked the first segment for a scheme, so
passing an already-absolute URL (e.g. from cart_url()) through the
|host filter would join route_prefix() + absolute URL, producing
"https://host/https://host/path/".
Now detects schemes in later segments and resets the base.
Also add missing 'market' entry to _nav.html _app_slugs to match
_nav_oob.html — without it the market menu item fell through to
coop_url('/market/') instead of market_url('/').
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>