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>

shared/sx/pages.py

@@ -219,7 +219,7 @@ async def execute_page(
     if page_def.menu_expr is not None:
         menu_sx = await _eval_slot(page_def.menu_expr, env, ctx, async_eval, async_eval_to_sx)
 
-    # Resolve layout → header rows
+    # Resolve layout → header rows + mobile menu fallback
     tctx = await get_template_context()
     header_rows = ""
     oob_headers = ""
@@ -261,6 +261,8 @@ async def execute_page(
         if layout is not None:
             header_rows = await layout.full_headers(tctx, **layout_kwargs)
             oob_headers = await layout.oob_headers(tctx, **layout_kwargs)
+            if not menu_sx:
+                menu_sx = await layout.mobile_menu(tctx, **layout_kwargs)
 
     # Branch on request type
     is_htmx = is_htmx_request()
@@ -288,17 +290,22 @@ async def execute_page(
 # Blueprint mounting
 # ---------------------------------------------------------------------------
 
-def mount_pages(bp: Any, service_name: str) -> None:
-    """Mount all registered PageDef routes onto a Quart Blueprint.
+def mount_pages(bp: Any, service_name: str,
+                names: set[str] | list[str] | None = None) -> None:
+    """Mount registered PageDef routes onto a Quart Blueprint.
 
     For each PageDef, adds a GET route with appropriate auth/cache
     decorators. Coexists with existing Python routes on the same blueprint.
+
+    If *names* is given, only mount pages whose name is in the set.
     """
     from quart import make_response
 
     pages = get_all_pages(service_name)
 
     for page_def in pages.values():
+        if names is not None and page_def.name not in names:
+            continue
         _mount_one_page(bp, service_name, page_def)
 
 
@@ -347,6 +354,9 @@ def _apply_auth(fn: Any, auth: str | list) -> Any:
     if auth == "admin":
         from shared.browser.app.authz import require_admin
         return require_admin(fn)
+    if auth == "post_author":
+        from shared.browser.app.authz import require_post_author
+        return require_post_author(fn)
     if isinstance(auth, list) and auth and auth[0] == "rights":
         from shared.browser.app.authz import require_rights
         return require_rights(*auth[1:])(fn)