Add delete endpoint with confirm modal, keep items at quantity 0

- POST /delete/<product_id>/ removes the cart item entirely
- POST /quantity/ now clamps at 0 instead of deleting
- cart_delete_url Jinja global registered for template

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

app.py

@@ -100,6 +100,7 @@ def create_app() -> "Quart":
     ])
 
     app.jinja_env.globals["cart_quantity_url"] = lambda product_id: f"/quantity/{product_id}/"
+    app.jinja_env.globals["cart_delete_url"] = lambda product_id: f"/delete/{product_id}/"
 
     # --- Page slug hydration (follows events/market app pattern) ---