Optional qBittorrent creds; require SABnzbd key

Allow qBittorrent to be configured without credentials (supports IP whitelist) and require an API key for SABnzbd. Skip connection testing when disabling a client. Updates include: validation changes in admin and setup API routes, test-download-client flows, DownloadClientModal UI validation and save/test logic, and DownloadClientManager to pass empty strings for optional credentials. Tests updated to reflect SABnzbd API key requirement.

src/app/api/admin/settings/download-clients/[id]/route.ts

@@ -77,12 +77,16 @@ export async function PUT(
           }
         }
 
-        // Test connection if credentials/URL changed
+        // Test connection if credentials/URL changed (skip if disabling client)
+        const isDisabling = enabled === false;
         if (
-          url !== undefined ||
-          username !== undefined ||
-          (password && password !== '********') ||
-          disableSSLVerify !== undefined
+          !isDisabling &&
+          (
+            url !== undefined ||
+            username !== undefined ||
+            (password && password !== '********') ||
+            disableSSLVerify !== undefined
+          )
         ) {
           const testResult = await manager.testConnection(updatedClient);
           if (!testResult.success) {