refactor(subscription): tighten payment flow boundaries

src/lib/navigation/subscription_exit.ts

@@ -0,0 +1,28 @@
+"use client";
+
+import { ROUTES } from "@/router/routes";
+
+import { consumePendingChatImageReturn } from "./chat_image_return_session";
+
+export type SubscriptionReturnTo = "chat" | null;
+
+export function consumeSubscriptionExplicitExitUrl(): string | null {
+  const pendingImageReturn = consumePendingChatImageReturn();
+  if (pendingImageReturn) return pendingImageReturn.returnUrl;
+  return null;
+}
+
+export function getSubscriptionFallbackExitUrl(
+  returnTo: SubscriptionReturnTo,
+): string {
+  return returnTo === "chat" ? ROUTES.chat : ROUTES.sidebar;
+}
+
+export function consumeSubscriptionExitUrl(
+  returnTo: SubscriptionReturnTo,
+): string {
+  return (
+    consumeSubscriptionExplicitExitUrl() ??
+    getSubscriptionFallbackExitUrl(returnTo)
+  );
+}