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>

federation/sx/notifications.sx

@@ -15,9 +15,6 @@
         preview
         (div :class "text-xs text-stone-400 mt-1" time)))))
 
-(defcomp ~federation-notifications-empty ()
-  (p :class "text-stone-500" "No notifications yet."))
-
 (defcomp ~federation-notifications-list (&key items)
   (div :class "space-y-2" items))