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/downloadClients.test.js

@@ -1,5 +1,5 @@
 // Copyright (c) 2026 Gordon Bolton. MIT License.
-const {
+import {
   DownloadClientRegistry,
   registry,
   initializeClients,
@@ -10,8 +10,8 @@ const {
   getDownloadsByClientType,
   testAllConnections,
   getAllClientStatuses
-} = require('../../server/utils/downloadClients');
-const { vi } = require('vitest');
+} from '../../server/utils/downloadClients.js';
+import { vi } from 'vitest';
 
 // Mock config and clients
 vi.mock('../../server/utils/config', () => ({