feat: persist customer product and shop favorites through the live API

Replace mall fixture favorites with customer-scoped endpoints, and send signed-out shoppers back to the page they left after sign-in.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/mall/pages/collective.vue

@@ -7,6 +7,7 @@ const { $api } = useNuxtApp();
 const session = useSessionStore();
 const cart = useCartStore();
 const router = useRouter();
+const signInThenReturn = useSignInRedirect();
 const activities = ref<GroupBuyingActivityView[]>([]);
 const loading = ref(true);
 const error = ref("");
@@ -42,7 +43,7 @@ function joined(activity: GroupBuyingActivityView, groupId: string): string {
 // intent; the server prices and validates it at checkout.
 async function start(activity: GroupBuyingActivityView, open: boolean): Promise<void> {
   if (!session.isLoggedIn) {
-    await navigateTo("/login");
+    await signInThenReturn();
     return;
   }
   error.value = "";