Fix examples.sx: paren balance + dict eval crash at startup

1. Extra closing paren in ex-tabs handler
2. tab-content dict values contained (div ...) HTML tags which crash
   during register_components since HTML primitives aren't in env

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

sx/sx/boundary.sx

@@ -114,3 +114,8 @@
   :params (filename title desc)
   :returns "dict"
   :service "sx")
+
+(define-page-helper "handler-source"
+  :params (name)
+  :returns "string"
+  :service "sx")