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>

market/sx/headers.sx

@@ -6,9 +6,6 @@
     (div :class "flex flex-col md:flex-row md:gap-2 text-xs"
       (div top-slug) sub-div)))
 
-(defcomp ~market-sub-slug (&key sub)
-  (div sub))
-
 (defcomp ~market-product-label (&key title)
   (<> (i :class "fa fa-shopping-bag" :aria-hidden "true") (div title)))