Consolidate post header/menu system into shared infrastructure

Replace duplicated _post_header_html, _oob_header_html, and header-child
components across blog/events/market/errors with shared sexpr components
(~post-label, ~page-cart-badge, ~oob-header, ~header-child, ~error-content)
and shared Python helpers (post_header_html, oob_header_html,
header_child_html, error_content_html). App-specific logic (blog container-nav
wrapping, admin cog, events calendar links) preserved via thin wrappers.

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

events/sexp/header.sexpr

@@ -1,20 +1,5 @@
 ;; Events header components
 
-(defcomp ~events-oob-header (&key parent-id child-id row-html)
-  (div :id parent-id :hx-swap-oob "outerHTML" :class "w-full"
-    (div :class "w-full"
-      (raw! row-html)
-      (div :id child-id))))
-
-(defcomp ~events-post-label (&key feature-image title)
-  (<> (when feature-image (img :src feature-image :class "h-8 w-8 rounded-full object-cover border border-stone-300 flex-shrink-0"))
-      (span title)))
-
-(defcomp ~events-post-cart-link (&key href count)
-  (a :href href :class "relative inline-flex items-center gap-1.5 px-3 py-1.5 text-sm rounded-full border border-emerald-300 bg-emerald-50 text-emerald-800 hover:bg-emerald-100 transition"
-    (i :class "fa fa-shopping-cart" :aria-hidden "true")
-    (span count)))
-
 (defcomp ~events-calendars-label ()
   (<> (i :class "fa fa-calendar" :aria-hidden "true") (div "Calendars")))
 
@@ -39,5 +24,3 @@
   (div :id (str "entry-title-" entry-id) :class "flex gap-1 items-center"
     (raw! title-html) (raw! times-html)))
 
-(defcomp ~events-header-child (&key inner-html)
-  (div :id "root-header-child" :class "w-full" (raw! inner-html)))