feat: Add 9 new features for enhanced UX and security

Quick Wins:
- Feature 1: Ticket linking in comments (#123456789 auto-links)
- Feature 6: Checkbox click area fix (click anywhere in cell)
- Feature 7: User groups display in settings modal

UI Enhancements:
- Feature 4: Collapsible sidebar with localStorage persistence
- Feature 5: Inline ticket preview popup on hover (300ms delay)
- Feature 2: Mobile responsive improvements (44px touch targets, iOS zoom fix)

Major Features:
- Feature 3: Kanban card view with status columns (toggle with localStorage)
- Feature 9: API key generation admin panel (/admin/api-keys)
- Feature 8: Ticket visibility levels (public/internal/confidential)

New files:
- views/admin/ApiKeysView.php
- api/generate_api_key.php
- api/revoke_api_key.php
- migrations/008_ticket_visibility.sql

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

index.php

@@ -219,6 +219,18 @@ switch (true) {
         include 'views/admin/AuditLogView.php';
         break;
 
+    case $requestPath == '/admin/api-keys':
+        if (!$currentUser || !$currentUser['is_admin']) {
+            header("HTTP/1.0 403 Forbidden");
+            echo 'Admin access required';
+            break;
+        }
+        require_once 'models/ApiKeyModel.php';
+        $apiKeyModel = new ApiKeyModel($conn);
+        $apiKeys = $apiKeyModel->getAllKeys();
+        include 'views/admin/ApiKeysView.php';
+        break;
+
     case $requestPath == '/admin/user-activity':
         if (!$currentUser || !$currentUser['is_admin']) {
             header("HTTP/1.0 403 Forbidden");