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>

account/app.py

@@ -72,9 +72,19 @@ def create_app() -> "Quart":
         app.jinja_loader,
     ])
 
+    # Setup defpage routes
+    import sx.sx_components  # noqa: F811 — ensure components loaded
+    from sxc.pages import setup_account_pages
+    setup_account_pages()
+
     # --- blueprints ---
     app.register_blueprint(register_auth_bp())
-    app.register_blueprint(register_account_bp())
+
+    account_bp = register_account_bp()
+    from shared.sx.pages import mount_pages
+    mount_pages(account_bp, "account")
+    app.register_blueprint(account_bp)
+
     app.register_blueprint(register_fragments())
 
     from bp.actions.routes import register as register_actions