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>

cart/app.py

@@ -52,17 +52,32 @@ async def cart_context() -> dict:
     """
     from shared.infrastructure.context import base_context
     from shared.services.navigation import get_navigation_tree
-    from shared.infrastructure.fragments import fetch_fragment
+    from shared.infrastructure.cart_identity import current_cart_identity
+    from shared.infrastructure.fragments import fetch_fragments
 
     ctx = await base_context()
 
-    ctx["nav_tree_html"] = await fetch_fragment(
-        "blog", "nav-tree",
-        params={"app_name": "cart", "path": request.path},
-    )
     # Fallback for _nav.html when nav-tree fragment fetch fails
     ctx["menu_items"] = await get_navigation_tree(g.s)
 
+    # Pre-fetch cross-app HTML fragments concurrently
+    user = getattr(g, "user", None)
+    ident = current_cart_identity()
+    cart_params = {}
+    if ident["user_id"] is not None:
+        cart_params["user_id"] = ident["user_id"]
+    if ident["session_id"] is not None:
+        cart_params["session_id"] = ident["session_id"]
+
+    cart_mini_html, auth_menu_html, nav_tree_html = await fetch_fragments([
+        ("cart", "cart-mini", cart_params or None),
+        ("account", "auth-menu", {"email": user.email} if user else None),
+        ("blog", "nav-tree", {"app_name": "cart", "path": request.path}),
+    ])
+    ctx["cart_mini_html"] = cart_mini_html
+    ctx["auth_menu_html"] = auth_menu_html
+    ctx["nav_tree_html"] = nav_tree_html
+
     # Cart app owns cart data — use g.cart from _load_cart
     all_cart = getattr(g, "cart", None) or []
     all_cal = await get_calendar_cart_entries(g.s)