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/ticket_dependencies.php

@@ -47,11 +47,16 @@ if ($conn->connect_error) {
     ResponseHelper::serverError('Database connection failed');
 }
 
-$dependencyModel = new DependencyModel($conn);
-$auditLog = new AuditLogModel($conn);
+try {
+    $dependencyModel = new DependencyModel($conn);
+    $auditLog = new AuditLogModel($conn);
+} catch (Exception $e) {
+    ResponseHelper::serverError('Failed to initialize models: ' . $e->getMessage());
+}
 
 $method = $_SERVER['REQUEST_METHOD'];
 
+try {
 switch ($method) {
     case 'GET':
         // Get dependencies for a ticket
@@ -139,5 +144,8 @@ switch ($method) {
     default:
         ResponseHelper::error('Method not allowed', 405);
 }
+} catch (Exception $e) {
+    ResponseHelper::serverError('An error occurred: ' . $e->getMessage());
+}
 
 $conn->close();