Add strikethrough support to markdown parser (~~text~~)

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

assets/js/markdown.js

@@ -41,6 +41,9 @@ function parseMarkdown(markdown) {
     html = html.replace(/\*(.+?)\*/g, '<em>$1</em>');
     html = html.replace(/_(.+?)_/g, '<em>$1</em>');
 
+    // Strikethrough (~~text~~)
+    html = html.replace(/~~(.+?)~~/g, '<del>$1</del>');
+
     // Images ![alt](url) - must come before link handler
     html = html.replace(/!\[([^\]]*)\]\(([^)]+)\)/g, function(match, alt, url) {
         if (/^https?:/i.test(url)) {