Add /quantity/ endpoint so cart +/- buttons work same-origin

Adds POST /quantity/<product_id>/ to set cart item quantity (or remove at 0),
and registers cart_quantity_url Jinja global so the shared template uses it.

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

app.py

@@ -99,6 +99,8 @@ def create_app() -> "Quart":
         app.jinja_loader,
     ])
 
+    app.jinja_env.globals["cart_quantity_url"] = lambda product_id: f"/quantity/{product_id}/"
+
     # --- Page slug hydration (follows events/market app pattern) ---
 
     @app.url_value_preprocessor