Fix cart template cross-app url_for crash and favicon 404

- Cart _cart.html: replace url_for('market.browse.product...') with
  market_product_url() for links and cart_global.update_quantity for
  quantity forms (market endpoints don't exist in cart app)
- Factory favicon route: use STATIC_DIR instead of relative "static"
  (resolves to shared/static/ where favicon.ico actually lives)
- Cart context processor: fetch all 3 fragments (cart-mini, auth-menu,
  nav-tree) concurrently, matching pattern in all other apps

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

shared/infrastructure/factory.py

@@ -284,6 +284,6 @@ def create_base_app(
     # --- favicon ---
     @app.get("/favicon.ico")
     async def favicon():
-        return await send_from_directory("static", "favicon.ico")
+        return await send_from_directory(STATIC_DIR, "favicon.ico")
 
     return app