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>

models/UserModel.php

@@ -266,4 +266,29 @@ class UserModel {
         $stmt->close();
         return $users;
     }
+
+    /**
+     * Get all distinct groups from all users
+     * Used for visibility group selection UI
+     *
+     * @return array Array of unique group names
+     */
+    public function getAllGroups() {
+        $stmt = $this->conn->prepare("SELECT DISTINCT groups FROM users WHERE groups IS NOT NULL AND groups != ''");
+        $stmt->execute();
+        $result = $stmt->get_result();
+
+        $allGroups = [];
+        while ($row = $result->fetch_assoc()) {
+            $userGroups = array_filter(array_map('trim', explode(',', $row['groups'])));
+            $allGroups = array_merge($allGroups, $userGroups);
+        }
+
+        $stmt->close();
+
+        // Return unique groups sorted alphabetically
+        $uniqueGroups = array_unique($allGroups);
+        sort($uniqueGroups);
+        return $uniqueGroups;
+    }
 }