fix(ci): scope Vitest to tests/unit so it doesn't collect Playwright specs

Vitest discovers any *.test.js / *.spec.js by default and tried to
import tests/e2e/marker-load.spec.js, where @playwright/test's test()
errors out because it's invoked outside the Playwright runner.

Author: kalwalt

vitest.config.js

@@ -0,0 +1,8 @@
+// vitest.config.js
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    include: ['tests/unit/**/*.test.js'],
+  },
+});