Refactor SX templates: shared components, Python migration, cleanup

- Extract shared components (empty-state, delete-btn, sentinel, crud-*,
  view-toggle, img-or-placeholder, avatar, sumup-settings-form, auth
  forms, order tables/detail/checkout)
- Migrate all Python sx_call() callers to use shared components directly
- Remove 55+ thin wrapper defcomps from domain .sx files
- Remove trivial passthrough wrappers (blog-header-label, market-card-text, etc)
- Unify duplicate auth flows (account + federation) into shared/sx/templates/auth.sx
- Unify duplicate order views (cart + orders) into shared/sx/templates/orders.sx
- Disable static file caching in dev (SEND_FILE_MAX_AGE_DEFAULT=0)
- Add SX response validation and debug headers

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

cart/sx/items.sx

@@ -3,10 +3,6 @@
 (defcomp ~cart-item-img (&key src alt)
   (img :src src :alt alt :class "w-24 h-24 sm:w-32 sm:h-28 object-cover rounded-xl border border-stone-100" :loading "lazy"))
 
-(defcomp ~cart-item-no-img ()
-  (div :class "w-24 h-24 sm:w-32 sm:h-28 rounded-xl border border-dashed border-stone-300 flex items-center justify-center text-xs text-stone-400"
-    "No image"))
-
 (defcomp ~cart-item-price (&key text)
   (p :class "text-sm sm:text-base font-semibold text-stone-900" text))
 
@@ -51,14 +47,6 @@
             (button :type "submit" :class "inline-flex items-center justify-center w-8 h-8 text-sm font-medium rounded-full border border-emerald-600 text-emerald-700 hover:bg-emerald-50 text-xl" "+")))
         (div :class "flex items-center justify-between sm:justify-end gap-3" (when line-total line-total))))))
 
-(defcomp ~cart-page-empty ()
-  (div :class "max-w-full px-3 py-3 space-y-3"
-    (div :id "cart"
-      (div :class "rounded-2xl border border-dashed border-stone-300 bg-white/80 p-6 sm:p-8 text-center"
-        (div :class "inline-flex h-10 w-10 sm:h-12 sm:w-12 items-center justify-center rounded-full bg-stone-100 mb-3"
-          (i :class "fa fa-shopping-cart text-stone-500 text-sm sm:text-base" :aria-hidden "true"))
-        (p :class "text-base sm:text-lg font-medium text-stone-800" "Your cart is empty")))))
-
 (defcomp ~cart-page-panel (&key items cal tickets summary)
   (div :class "max-w-full px-3 py-3 space-y-3"
     (div :id "cart"