fix: hide unavailable Twins Lab demo

lib/site-content.test.ts

@@ -98,3 +98,9 @@ it("resolves every declared related case ID", () => {
 
   expect(unresolved).toEqual([]);
 });
+
+it("does not publish the unavailable Twins Lab demo", () => {
+  expect(
+    caseStudies.find((item) => item.id === "twins-lab")?.demoUrl,
+  ).toBeUndefined();
+});