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/cart/overview_routes.py

@@ -1,31 +1,26 @@
 # bp/cart/overview_routes.py — Cart overview (list of page carts)
+# GET / handled by defpage.
 
 from __future__ import annotations
 
-from quart import Blueprint, render_template, make_response
+from quart import Blueprint, g, request
 
-from shared.browser.app.utils.htmx import is_htmx_request
-from shared.sx.helpers import sx_response
 from .services import get_cart_grouped_by_page
 
 
 def register(url_prefix: str) -> Blueprint:
     bp = Blueprint("cart_overview", __name__, url_prefix=url_prefix)
 
-    @bp.get("/")
-    async def overview():
-        from quart import g
+    @bp.before_request
+    async def _prepare_page_data():
+        """Load overview data for defpage route."""
+        endpoint = request.endpoint or ""
+        if not endpoint.endswith("defpage_cart_overview"):
+            return
         from shared.sx.page import get_template_context
-        from sx.sx_components import render_overview_page, render_overview_oob
-
+        from sx.sx_components import _overview_main_panel_sx
         page_groups = await get_cart_grouped_by_page(g.s)
         ctx = await get_template_context()
-
-        if not is_htmx_request():
-            html = await render_overview_page(ctx, page_groups)
-            return await make_response(html)
-        else:
-            sx_src = await render_overview_oob(ctx, page_groups)
-            return sx_response(sx_src)
+        g.overview_content = _overview_main_panel_sx(page_groups, ctx)
 
     return bp