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>

market/bp/page_admin/routes.py

@@ -6,7 +6,6 @@ import unicodedata
 from quart import make_response, request, g, Blueprint
 
 from shared.browser.app.authz import require_admin
-from shared.browser.app.utils.htmx import is_htmx_request
 from shared.services.registry import services
 from shared.sx.helpers import sx_response
 
@@ -27,19 +26,16 @@ def _slugify(value: str, max_len: int = 255) -> str:
 def register():
     bp = Blueprint("page_admin", __name__)
 
-    @bp.get("/")
-    @require_admin
-    async def admin(**kwargs):
+    @bp.before_request
+    async def _prepare_page_data():
+        """Pre-render page admin content for defpage (async helper)."""
+        endpoint = request.endpoint or ""
+        if request.method != "GET" or not endpoint.endswith("defpage_page_admin"):
+            return
         from shared.sx.page import get_template_context
-        from sx.sx_components import render_page_admin_page, render_page_admin_oob
-
-        tctx = await get_template_context()
-        if not is_htmx_request():
-            html = await render_page_admin_page(tctx)
-            return await make_response(html)
-        else:
-            sx_src = await render_page_admin_oob(tctx)
-            return sx_response(sx_src)
+        from sx.sx_components import _markets_admin_panel_sx
+        ctx = await get_template_context()
+        g.page_admin_content = await _markets_admin_panel_sx(ctx)
 
     @bp.post("/new/")
     @require_admin