Improve SABnzbd API key handling and settings UI

Clear credentials when switching download client types in the admin settings page and update validation logic for enabling the test connection button. In SABnzbd service, trim API key input and add explicit validation and error handling for missing or invalid API keys during connection tests.

src/app/admin/settings/page.tsx

@@ -1532,9 +1532,15 @@ export default function AdminSettings() {
                   <select
                     value={settings.downloadClient.type}
                     onChange={(e) => {
+                      // Clear credentials when switching client types
                       setSettings({
                         ...settings,
-                        downloadClient: { ...settings.downloadClient, type: e.target.value },
+                        downloadClient: {
+                          ...settings.downloadClient,
+                          type: e.target.value,
+                          username: '', // Clear username (only used by qBittorrent)
+                          password: '', // Clear password/API key
+                        },
                       });
                       setValidated({ ...validated, download: false });
                     }}
@@ -1775,7 +1781,11 @@ export default function AdminSettings() {
                   <Button
                     onClick={testDownloadClientConnection}
                     loading={testing}
-                    disabled={!settings.downloadClient.url || !settings.downloadClient.username || !settings.downloadClient.password}
+                    disabled={
+                      !settings.downloadClient.url ||
+                      !settings.downloadClient.password ||
+                      (settings.downloadClient.type === 'qbittorrent' && !settings.downloadClient.username)
+                    }
                     variant="outline"
                     className="w-full"
                   >