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

eleventy.config.js

@@ -223,6 +223,9 @@ export default function (eleventyConfig) {
     }
   });
 
+  // Current timestamp filter (for client-side JS buildtime)
+  eleventyConfig.addFilter("timestamp", () => Date.now());
+
   // Date filter (for sidebar dates)
   eleventyConfig.addFilter("date", (dateObj, format) => {
     if (!dateObj) return "";