Add comment threading and fix fetch authentication

- Add comment threading/reply functionality with nested display
  - Database migration for parent_comment_id and thread_depth columns
  - Recursive comment rendering with depth-based indentation
  - Reply form with inline UI and smooth animations
  - Thread collapse/expand capability
  - Max thread depth of 3 levels

- Fix 401 authentication errors on API calls
  - Add credentials: 'same-origin' to all fetch calls
  - Affects settings.js, ticket.js, dashboard.js, advanced-search.js
  - Ensures session cookies are sent with requests

- Enhanced comment styling
  - Thread connector lines for visual hierarchy
  - Reply button on comments (up to depth 3)
  - Quote block styling for replies

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

assets/js/settings.js

@@ -8,7 +8,9 @@ let userPreferences = {};
 // Load preferences on page load
 async function loadUserPreferences() {
     try {
-        const response = await fetch('/api/user_preferences.php');
+        const response = await fetch('/api/user_preferences.php', {
+            credentials: 'same-origin'
+        });
         const data = await response.json();
         if (data.success) {
             userPreferences = data.preferences;
@@ -96,6 +98,7 @@ async function saveSettings() {
         for (const [key, value] of Object.entries(prefs)) {
             const response = await fetch('/api/user_preferences.php', {
                 method: 'POST',
+                credentials: 'same-origin',
                 headers: {
                     'Content-Type': 'application/json',
                     'X-CSRF-Token': window.CSRF_TOKEN