feat: extract shared auth utility and fix custom provider model resolution

src/app/(dashboard)/dashboard/providers/[id]/page.tsx

@@ -1511,7 +1511,7 @@ function CompatibleModelsSection({
       });
 
       if (!customModelRes.ok) {
-        let errorData = {};
+        let errorData: { error?: { message?: string } } = {};
         try {
           errorData = await customModelRes.json();
         } catch (jsonError) {
@@ -1519,7 +1519,6 @@ function CompatibleModelsSection({
         }
         throw new Error(errorData.error?.message || "Failed to save custom model");
       }
-      }
 
       // Only create alias after customModel is saved successfully
       await onSetAlias(modelId, resolvedAlias, providerStorageAlias);