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/utils/auth.ts

@@ -0,0 +1,11 @@
+/**
+ * Sign-in URL that returns the shopper to `fullPath` afterwards. `login.vue`
+ * only honours a same-origin absolute path, and a redirect back to the sign-in
+ * page itself would loop, so that case drops the parameter.
+ */
+export function signInPath(fullPath: string): string {
+  if (!fullPath.startsWith("/") || fullPath.startsWith("//") || fullPath.startsWith("/login")) {
+    return "/login";
+  }
+  return `/login?redirect=${encodeURIComponent(fullPath)}`;
+}