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/templates/layout.sx

@@ -120,6 +120,31 @@
 (defcomp ~header-child-sx (&key id inner)
   (div :id (or id "root-header-child") :class "flex flex-col w-full items-center" inner))
 
+;; ---------------------------------------------------------------------------
+;; Mobile menu — vertical nav sections for hamburger panel
+;; ---------------------------------------------------------------------------
+
+;; Labelled section: colour bar header + vertical nav items
+(defcomp ~mobile-menu-section (&key label href colour level items)
+  (let* ((c (or colour "sky"))
+         (lv (or level 1))
+         (shade (str (- 500 (* lv 100)))))
+    (div
+      (div :class (str "flex items-center gap-2 px-3 py-1.5 text-sm font-bold bg-" c "-" shade)
+        (if href
+          (a :href href :class "hover:underline" label)
+          (span label)))
+      (div :class "flex flex-col gap-1 p-2 text-sm"
+        items))))
+
+;; Root-level mobile nav: site nav items + auth links
+(defcomp ~mobile-root-nav (&key nav-tree auth-menu)
+  (<>
+    (when nav-tree
+      (div :class "flex flex-col gap-2 p-3 text-sm" nav-tree))
+    (when auth-menu
+      (div :class "p-3 border-t border-stone-200" auth-menu))))
+
 (defcomp ~error-content (&key errnum message image)
   (div :class "text-center p-8 max-w-lg mx-auto"
     (div :class "font-bold text-2xl md:text-4xl text-red-500 mb-4" errnum)