fix: convert test files to ES modules and fix qbittorrent test method calls

- Convert all client test files from CommonJS require() to ES module import syntax
- Convert downloadClients.test.js and integration/downloadClients.test.js to ES modules
- Fix qbittorrent.test.js to use getActiveDownloads() instead of getTorrents()
- All test files now use proper Vitest-compatible ES module syntax
- Resolves Vitest import errors and QBittorrentClient method call errors

tests/unit/clients/TransmissionClient.test.js

@@ -1,7 +1,7 @@
 // Copyright (c) 2026 Gordon Bolton. MIT License.
-const TransmissionClient = require('../../../server/clients/TransmissionClient');
-const axios = require('axios');
-const { vi } = require('vitest');
+import TransmissionClient from '../../../server/clients/TransmissionClient.js';
+import axios from 'axios';
+import { vi } from 'vitest';
 
 // Mock axios
 vi.mock('axios');