fix: integration fixes from final review

- useSettings: upsert instead of update (safe if migration rows missing)
- invite-admin-user: return created_at and last_sign_in_at in response
- UserManagementPage: clear deleteError when opening new confirmation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

supabase/functions/invite-admin-user/index.ts

@@ -60,7 +60,12 @@ Deno.serve(async (req) => {
   }
 
   return new Response(
-    JSON.stringify({ id: data.user.id, email: data.user.email }),
+    JSON.stringify({
+      id: data.user.id,
+      email: data.user.email,
+      created_at: data.user.created_at,
+      last_sign_in_at: data.user.last_sign_in_at ?? null,
+    }),
     { headers: { ...corsHeaders, 'Content-Type': 'application/json' } },
   );
 });