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/bp/page_admin/routes.py

@@ -7,42 +7,28 @@ from quart import (
 from shared.infrastructure.actions import call_action
 from shared.infrastructure.data_client import fetch_data
 from shared.browser.app.authz import require_admin
-from shared.browser.app.utils.htmx import is_htmx_request
 from shared.sx.helpers import sx_response
 
 
 def register():
     bp = Blueprint("page_admin", __name__)
 
-    @bp.get("/")
-    @require_admin
-    async def admin(**kwargs):
-        from shared.sx.page import get_template_context
-        from sx.sx_components import render_cart_admin_page, render_cart_admin_oob
-
-        ctx = await get_template_context()
-        page_post = getattr(g, "page_post", None)
-        if not is_htmx_request():
-            html = await render_cart_admin_page(ctx, page_post)
-            return await make_response(html)
-        else:
-            sx_src = await render_cart_admin_oob(ctx, page_post)
-            return sx_response(sx_src)
-
-    @bp.get("/payments/")
-    @require_admin
-    async def payments(**kwargs):
-        from shared.sx.page import get_template_context
-        from sx.sx_components import render_cart_payments_page, render_cart_payments_oob
-
-        ctx = await get_template_context()
-        page_post = getattr(g, "page_post", None)
-        if not is_htmx_request():
-            html = await render_cart_payments_page(ctx, page_post)
-            return await make_response(html)
-        else:
-            sx_src = await render_cart_payments_oob(ctx, page_post)
-            return sx_response(sx_src)
+    @bp.before_request
+    async def _prepare_page_data():
+        """Pre-render admin content for defpage routes."""
+        endpoint = request.endpoint or ""
+        if request.method != "GET":
+            return
+        if endpoint.endswith("defpage_cart_admin"):
+            from shared.sx.page import get_template_context
+            from sx.sx_components import _cart_admin_main_panel_sx
+            ctx = await get_template_context()
+            g.cart_admin_content = _cart_admin_main_panel_sx(ctx)
+        elif endpoint.endswith("defpage_cart_payments"):
+            from shared.sx.page import get_template_context
+            from sx.sx_components import _cart_payments_main_panel_sx
+            ctx = await get_template_context()
+            g.cart_payments_content = _cart_payments_main_panel_sx(ctx)
 
     @bp.put("/payments/")
     @require_admin