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>

account/sx/newsletters.sx

@@ -10,12 +10,6 @@
       :class cls :role "switch" :aria-checked checked
       (span :class knob-cls))))
 
-(defcomp ~account-newsletter-toggle-off (&key id url hdrs target)
-  (div :id id :class "flex items-center"
-    (button :sx-post url :sx-headers hdrs :sx-target target :sx-swap "outerHTML"
-      :class "relative inline-flex h-6 w-11 items-center rounded-full transition-colors focus:outline-none focus:ring-2 focus:ring-emerald-500 focus:ring-offset-2 bg-stone-300"
-      :role "switch" :aria-checked "false"
-      (span :class "inline-block h-4 w-4 rounded-full bg-white shadow transform transition-transform translate-x-1"))))
 
 (defcomp ~account-newsletter-item (&key name desc toggle)
   (div :class "flex items-center justify-between py-4 first:pt-0 last:pb-0"