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>

templates/fragments/nav_tree.html

@@ -2,7 +2,13 @@
    Uses frag_app_name / frag_first_seg instead of request.path / app_name
    so the consuming app's context is reflected correctly.
    No hx-boost — cross-app nav links are full page navigations. #}
-{% set _app_slugs = {'cart': cart_url('/')} %}
+{% set _app_slugs = {
+  'cart': cart_url('/'),
+  'market': market_url('/'),
+  'events': events_url('/'),
+  'federation': federation_url('/'),
+  'account': account_url('/'),
+} %}
 <div class="flex flex-col sm:flex-row sm:items-center gap-2 border-r border-stone-200 mr-2 sm:max-w-2xl"
      id="menu-items-nav-wrapper">
   {% from 'macros/scrolling_menu.html' import scrolling_menu with context %}