Add OOB header swaps for sx docs navigation + enable OAuth + fragments

- OOB nav updates: AJAX navigation now swaps both menu bar levels
  (main nav highlighting + sub-nav with current page) using the same
  oob_header_sx/oob_page_sx pattern as blog/market/events
- Enable OAuth for sx and test apps (removed from _NO_OAUTH, added sx
  to ALLOWED_CLIENTS, added app_urls for sx/test/orders)
- Fetch real cross-service fragments (cart-mini, auth-menu, nav-tree)
  instead of hardcoding empty values
- Add :selected param to ~menu-row-sx for white text current-page label
- Fix duplicate element IDs: use menu-row-sx child_id/child mechanism
  instead of manual header_child_sx wrappers
- Fix home page copy: "Server-rendered DOM over the wire (no HTML)"

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

_config/app-config.yaml

@@ -16,6 +16,9 @@ app_urls:
   events: "https://events.rose-ash.com"
   federation: "https://federation.rose-ash.com"
   account: "https://account.rose-ash.com"
+  sx: "https://sx.rose-ash.com"
+  test: "https://test.rose-ash.com"
+  orders: "https://orders.rose-ash.com"
 cache:
   fs_root: /app/_snapshot  # <- absolute path to your snapshot dir
 categories: