Migrate all apps to defpage declarative page routes

Replace Python GET page handlers with declarative defpage definitions in .sx
files across all 8 apps (sx docs, orders, account, market, cart, federation,
events, blog). Each app now has sxc/pages/ with setup functions, layout
registrations, page helpers, and .sx defpage declarations.

Core infrastructure: add g I/O primitive, PageDef support for auth/layout/
data/content/filter/aside/menu slots, post_author auth level, and custom
layout registration. Remove ~1400 lines of render_*_page/render_*_oob
boilerplate. Update all endpoint references in routes, sx_components, and
templates to defpage_* naming.

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

sx/app.py

@@ -50,7 +50,15 @@ def create_app() -> "Quart":
 
     import sxc.sx_components  # noqa: F401
 
-    app.register_blueprint(register_pages(url_prefix="/"))
+    from sxc.pages import setup_sx_pages
+    setup_sx_pages()
+
+    bp = register_pages(url_prefix="/")
+
+    from shared.sx.pages import mount_pages
+    mount_pages(bp, "sx")
+
+    app.register_blueprint(bp)
 
     return app