fix(providers): label missing authIndex as not-set instead of unavailable

The detail metadata section was rendering authIndex with the
status.notAvailable label (zh: "不可用"), which reads like the
provider doesn't support the feature at all. Every provider supports
authIndex; it's just absent until you link an auth file. Reuse the
status.notSet label so the wording matches baseUrl / proxyUrl and
makes it clear that the entry simply doesn't have a value yet.

src/features/providers/sheets/ResourceDetailView.tsx

@@ -20,7 +20,7 @@ export function ResourceDetailView({ resource }: ResourceDetailViewProps) {
   ];
 
   const metadata: Array<[string, string]> = [
-    ['authIndex', resource.authIndex ?? t('providersPage.status.notAvailable')],
+    ['authIndex', resource.authIndex ?? t('providersPage.status.notSet')],
     ['excludedModels', String(resource.excludedModelCount)],
     ['apiKeyEntries', String(resource.apiKeyEntryCount)],
   ];