Fix markdown CSS not applying to comments — add lt-markdown class

comment-text divs with data-markdown were never getting the lt-markdown
class, so all scoped CSS (ul/ol/li bullets, mark, del, task items, etc.)
had no effect. Fixed in PHP template, JS comment builder, and
renderMarkdownComments().

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

assets/js/markdown.js

@@ -516,6 +516,7 @@ function processPlainTextComments() {
  */
 function renderMarkdownComments() {
     document.querySelectorAll('.comment-text[data-markdown]:not([data-rendered])').forEach(el => {
+        el.classList.add('lt-markdown');
         el.innerHTML = parseMarkdown(el.textContent);
         el.dataset.rendered = '1';
     });