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>

assets/js/markdown.js

@@ -13,6 +13,9 @@ function parseMarkdown(markdown) {
                .replace(/</g, '&lt;')
                .replace(/>/g, '&gt;');
 
+    // Ticket references (#123456789) - convert to clickable links
+    html = html.replace(/#(\d{9})\b/g, '<a href="/ticket/$1" class="ticket-link-ref">#$1</a>');
+
     // Code blocks (```code```)
     html = html.replace(/```([\s\S]*?)```/g, '<pre class="code-block"><code>$1</code></pre>');