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/upload_attachment.php

@@ -31,8 +31,8 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
         ResponseHelper::error('Ticket ID is required');
     }
 
-    // Validate ticket ID format
-    if (!preg_match('/^[A-Z]{3}\d{6}$/', $ticketId)) {
+    // Validate ticket ID format (9-digit number)
+    if (!preg_match('/^\d{9}$/', $ticketId)) {
         ResponseHelper::error('Invalid ticket ID format');
     }
 
@@ -72,8 +72,8 @@ if (empty($ticketId)) {
     ResponseHelper::error('Ticket ID is required');
 }
 
-// Validate ticket ID format
-if (!preg_match('/^[A-Z]{3}\d{6}$/', $ticketId)) {
+// Validate ticket ID format (9-digit number)
+if (!preg_match('/^\d{9}$/', $ticketId)) {
     ResponseHelper::error('Invalid ticket ID format');
 }