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>

cart/app.py

@@ -181,6 +181,12 @@ def create_app() -> "Quart":
         )
         g.page_config = _make_page_config(raw_pc) if raw_pc else None
 
+    # Setup defpage routes
+    from sxc.pages import setup_cart_pages
+    setup_cart_pages()
+
+    from shared.sx.pages import mount_pages
+
     # --- Blueprint registration ---
     # Static prefixes first, dynamic (page_slug) last
 
@@ -191,22 +197,19 @@ def create_app() -> "Quart":
     )
 
     # Cart overview at GET /
-    app.register_blueprint(
-        register_cart_overview(url_prefix="/"),
-        url_prefix="/",
-    )
+    overview_bp = register_cart_overview(url_prefix="/")
+    mount_pages(overview_bp, "cart", names=["cart-overview"])
+    app.register_blueprint(overview_bp, url_prefix="/")
 
     # Page admin at /<page_slug>/admin/ (before page_cart catch-all)
-    app.register_blueprint(
-        register_page_admin(),
-        url_prefix="/<page_slug>/admin",
-    )
+    admin_bp = register_page_admin()
+    mount_pages(admin_bp, "cart", names=["cart-admin", "cart-payments"])
+    app.register_blueprint(admin_bp, url_prefix="/<page_slug>/admin")
 
     # Page cart at /<page_slug>/ (dynamic, matched last)
-    app.register_blueprint(
-        register_page_cart(url_prefix="/"),
-        url_prefix="/<page_slug>",
-    )
+    page_cart_bp = register_page_cart(url_prefix="/")
+    mount_pages(page_cart_bp, "cart", names=["page-cart-view"])
+    app.register_blueprint(page_cart_bp, url_prefix="/<page_slug>")
 
     return app