Remove cross-DB menu_nodes query from non-blog apps

menu_nodes table lives in db_blog. Non-blog apps (cart, market, events,
account, federation) were querying it on their own DB session, causing
UndefinedTableError. The nav-tree fragment from blog provides the real
navigation; menu_items is now an empty fallback list.

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

cart/app.py

@@ -54,14 +54,13 @@ async def cart_context() -> dict:
     Global cart_count / cart_total stay global for cart-mini.
     """
     from shared.infrastructure.context import base_context
-    from shared.services.navigation import get_navigation_tree
     from shared.infrastructure.cart_identity import current_cart_identity
     from shared.infrastructure.fragments import fetch_fragments
 
     ctx = await base_context()
 
-    # Fallback for _nav.html when nav-tree fragment fetch fails
-    ctx["menu_items"] = await get_navigation_tree(g.s)
+    # menu_nodes lives in db_blog; nav-tree fragment provides the real nav
+    ctx["menu_items"] = []
 
     # Pre-fetch cross-app HTML fragments concurrently
     user = getattr(g, "user", None)