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>

shared/infrastructure/factory.py

@@ -145,8 +145,8 @@ def create_base_app(
     errors(app)
 
     # Auto-register OAuth client blueprint for non-account apps
-    # (account is the OAuth authorization server; test/sx are public dashboards)
-    _NO_OAUTH = {"account", "test", "sx"}
+    # (account is the OAuth authorization server)
+    _NO_OAUTH = {"account"}
     if name not in _NO_OAUTH:
         from shared.infrastructure.oauth import create_oauth_blueprint
         app.register_blueprint(create_oauth_blueprint(name))