Add multi-download-client support and UI management

Implements support for configuring both qBittorrent and SABnzbd simultaneously, including migration from legacy config, protocol-aware routing, and protocol filtering. Adds new CRUD API routes for download clients, new UI management components, and updates setup and settings flows to use the new multi-client architecture. Updates documentation to describe the new structure and usage.

tests/components/ui/Modal.test.tsx

@@ -26,6 +26,6 @@ describe('Modal', () => {
     expect(onClose).toHaveBeenCalledTimes(1);
 
     unmount();
-    expect(document.body.style.overflow).toBe('unset');
+    expect(document.body.style.overflow).toBe(''); // Cleared to default
   });
 });