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/composables/useSignInRedirect.ts

@@ -0,0 +1,10 @@
+import { signInPath } from "~/utils/auth";
+
+/** Send a signed-out shopper to sign in and back to the page they were on. */
+export function useSignInRedirect() {
+  const route = useRoute();
+  const router = useRouter();
+  return async function signInThenReturn(): Promise<void> {
+    await router.push(signInPath(route.fullPath));
+  };
+}