fix: remove trailing slash from POST /cart route

Templates generate URLs without trailing slash via market_url(), but
the route required one. POST requests don't get redirected by Werkzeug,
causing a 405 MethodNotAllowed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bp/product/routes.py

@@ -157,7 +157,7 @@ def register():
     from models.market import CartItem
     from quart import request, url_for
 
-    @bp.post("/cart/")
+    @bp.post("/cart")
     @clear_cache(tag="browse", tag_scope="user")
     async def cart(slug: str):
         # make sure product exists (we *allow* deleted_at != None later if you want)