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>

account/bp/auth/routes.py

@@ -44,7 +44,7 @@ from .services import (
 SESSION_USER_KEY = "uid"
 ACCOUNT_SESSION_KEY = "account_sid"
 
-ALLOWED_CLIENTS = {"blog", "market", "cart", "events", "federation", "orders", "test", "artdag", "artdag_l2"}
+ALLOWED_CLIENTS = {"blog", "market", "cart", "events", "federation", "orders", "test", "sx", "artdag", "artdag_l2"}
 
 
 def register(url_prefix="/auth"):