Fix doubled URLs when |host filter receives absolute URLs

_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>

utils.py

@@ -39,7 +39,12 @@ def _join_url_parts(parts: List[str]) -> str:
     cleaned = [first.strip("/")]
     for seg in parts[1:]:
         seg = str(seg)
-        if seg.startswith("?") or seg.startswith("#"):
+        # If a later segment is already an absolute URL, use it as the base
+        m2 = re.match(r"^([a-zA-Z][a-zA-Z0-9+.-]*://)(.*)$", seg)
+        if m2:
+            scheme, first = m2.group(1), m2.group(2)
+            cleaned = [first.strip("/")]
+        elif seg.startswith("?") or seg.startswith("#"):
             cleaned[-1] = cleaned[-1] + seg  # attach query/fragment
         else:
             cleaned.append(seg.strip("/"))