feat(storybook): wire @storybook/test-runner for story smoke tests

Every story is now executed as a smoke test (mount + no console errors)
via @storybook/test-runner. New script: pnpm test:stories runs
build-storybook then test-storybook against the static build.

Spec: §6.8

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/storybook/test-runner.config.ts

@@ -0,0 +1,13 @@
+import type { TestRunnerConfig } from "@storybook/test-runner";
+
+const config: TestRunnerConfig = {
+  async preVisit(page) {
+    page.on("console", (msg) => {
+      if (msg.type() === "error") {
+        throw new Error(`Console error in story: ${msg.text()}`);
+      }
+    });
+  },
+};
+
+export default config;