fix: use timestamp filter for buildtime instead of date filter

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

_includes/components/webmentions.njk

@@ -5,7 +5,7 @@
 
 {% set mentions = webmentions | webmentionsForUrl(page.url, urlAliases) %}
 {% set absoluteUrl = site.url + page.url %}
-{% set buildTimestamp = "now" | date("x") %}
+{% set buildTimestamp = "" | timestamp %}
 
 {# Data container for client-side JS to fetch new webmentions #}
 <div data-webmentions