Restore menu_items fallback, fix app slug URLs in nav fragment

Keep get_navigation_tree() as fallback when nav-tree fragment fetch
fails. Also map all app slugs (market, events, federation, account)
to their proper cross-app URLs in the nav fragment template.

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

app.py

@@ -27,12 +27,16 @@ async def blog_context() -> dict:
     - cart_mini_html / auth_menu_html / nav_tree_html: pre-fetched fragments
     """
     from shared.infrastructure.context import base_context
+    from shared.services.navigation import get_navigation_tree
     from shared.services.registry import services
     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)
+
     # Cart data via service (replaces cross-app HTTP API)
     ident = current_cart_identity()
     summary = await services.cart.cart_summary(