Add test-runner config to inject __test global for Storybook smoke tests

.storybook/test-runner.ts

@@ -0,0 +1,23 @@
+import type { TestRunnerConfig } from '@storybook/test-runner';
+
+// Extend Window interface to include __test property
+// Note: This variable is referenced in test-runner's page.evaluate calls
+// but not directly in our codebase. It appears to be expected by the test environment.
+declare global {
+  interface Window {
+    __test?: boolean;
+  }
+}
+
+const config: TestRunnerConfig = {
+  async preVisit(page) {
+    // Inject __test global to prevent ReferenceError during test execution
+    // This addresses the error: "page.evaluate: ReferenceError: __test is not defined"
+    // that occurs when running Storybook test-runner smoke tests
+    await page.evaluate(() => {
+      window.__test = true;
+    });
+  },
+};
+
+export default config;