fix: Resolve multiple UI and API bugs

- Remove is_active filter from get_users.php (column doesn't exist)
- Fix ticket ID validation regex in upload_attachment.php (9-digit format)
- Fix createSettingsModal reference to use openSettingsModal from settings.js
- Add error handling for dependencies tab to prevent infinite loading
- Add try-catch wrapper to ticket_dependencies.php API
- Make export dropdown visible only when tickets are selected
- Export only selected tickets instead of all filtered tickets

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

api/get_users.php

@@ -34,8 +34,8 @@ try {
 
     header('Content-Type: application/json');
 
-    // Get all active users for mentions
-    $sql = "SELECT user_id, username, display_name FROM users WHERE is_active = 1 ORDER BY display_name, username";
+    // Get all users for mentions/assignment
+    $sql = "SELECT user_id, username, display_name FROM users ORDER BY display_name, username";
     $result = $conn->query($sql);
 
     $users = [];