update from user testing payment platform

frontend/src/routes/[fam]/+layout.svelte

@@ -36,7 +36,6 @@
 	function handleCheckoutReturn() {
 		if (returnHandled || page.url.searchParams.get('checkout') !== 'return') return;
 		returnHandled = true;
-		history.replaceState({}, '', page.url.pathname); // cosmetic only
 		if (!disabled) {
 			notices.success('Welcome to FamChore!', 'Your subscription is active.');
 			return;
@@ -81,7 +80,11 @@
 
 	onDestroy(stopActivating);
 
-	handleCheckoutReturn();
+	// Client-only: touches history/notices — must not run during SSR
+	// (history.replaceState throws server-side → 500 on ?checkout=return).
+	$effect(() => {
+		handleCheckoutReturn();
+	});
 
 	function accessMessage(reason: string, parent: boolean) {
 		const map: Record<string, string> = {