fix: register cart loader so g.cart is available for product cart route

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

app.py

@@ -8,6 +8,7 @@ from jinja2 import FileSystemLoader, ChoiceLoader
 from sqlalchemy import select
 
 from shared.factory import create_base_app
+from shared.cart_loader import load_cart
 from config import config
 
 from suma_browser.app.bp import register_market_bp
@@ -45,7 +46,7 @@ def create_app() -> "Quart":
     from models.market_place import MarketPlace
     from models.ghost_content import Post
 
-    app = create_base_app("market", context_fn=market_context)
+    app = create_base_app("market", context_fn=market_context, before_request_fns=[load_cart])
 
     # App-specific templates override shared templates
     app_templates = str(Path(__file__).resolve().parent / "templates")