Rebrand sexp → sx across web platform (173 files)

Rename all sexp directories, files, identifiers, and references to sx.
artdag/ excluded (separate media processing DSL).

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

federation/sx/notifications.sx

@@ -0,0 +1,25 @@
+;; Notification components
+
+(defcomp ~federation-notification-preview (&key preview)
+  (div :class "text-sm text-stone-500 mt-1 truncate" preview))
+
+(defcomp ~federation-notification-card (&key cls avatar from-name from-username from-domain action-text preview time)
+  (div :class cls
+    (div :class "flex items-start gap-3"
+      avatar
+      (div :class "flex-1"
+        (div :class "text-sm"
+          (span :class "font-semibold" from-name)
+          " " (span :class "text-stone-500" "@" from-username from-domain)
+          " " (span :class "text-stone-600" action-text))
+        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))
+
+(defcomp ~federation-notifications-page (&key notifs)
+  (h1 :class "text-2xl font-bold mb-6" "Notifications") notifs)