Replace env free-variable threading with IO-primitive auto-fetch macros

Layout components now self-resolve context (cart-mini, auth-menu, nav-tree,
rights, URLs) via new IO primitives (root-header-ctx, select-colours,
account-nav-ctx, app-rights) and defmacro wrappers (~root-header-auto,
~auth-header-row-auto, ~root-mobile-auto). This eliminates _ctx_to_env(),
HELPER_CSS_CLASSES, and verbose :key threading across all 10 services.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

market/sx/headers.sx

@@ -4,7 +4,7 @@
   (div :class "font-bold text-xl flex-shrink-0 flex gap-2 items-center"
     (div (i :class "fa fa-shop") " " title)
     (div :class "flex flex-col md:flex-row md:gap-2 text-xs"
-      (div top-slug) sub-div)))
+      (div top-slug) (when sub-div (div sub-div)))))
 
 (defcomp ~market-product-label (&key title)
   (<> (i :class "fa fa-shopping-bag" :aria-hidden "true") (div title)))