security: OWASP audit and hardening (v1.0.8)

- Archive XSS: wrap get_the_archive_title/description with wp_kses_post()
  in ContextBuilder to sanitize Editor-editable term content rendered via |raw
- Comment fields: esc_html() on comment_author, esc_url() on comment_author_url
  at data source; template updated to output pre-escaped URL via |raw
- dark-mode.js: whitelist localStorage value against ['dark','light'] to
  prevent attribute injection from third-party script tampering
- TwigService: add is_safe=>html to esc_html/esc_attr/esc_url Twig functions
  to prevent double-encoding if autoescape is ever enabled
- Add .markdownlint.json (disable MD024 duplicate headings, MD013 line length)

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

src/js/dark-mode.js

@@ -20,7 +20,9 @@
      */
     function getPreferredTheme() {
         var stored = localStorage.getItem(STORAGE_KEY);
-        if (stored) {
+        // Whitelist: only honour known-good values to prevent attribute injection
+        // from a tampered localStorage (e.g. XSS-written value by another script).
+        if (stored === 'dark' || stored === 'light') {
             return stored;
         }
         return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';