test(dsh-plugin-browserskill): build the scoped css before vitest

The client entry imports the generated bsk-ui.nomodule.css; the file is
gitignored, so a fresh CI checkout could not resolve it. Generate it as part
of the test script.

packages/dsh-plugin-browserskill/package.json

@@ -36,7 +36,7 @@
   "scripts": {
     "build": "node scripts/build-client-css.mjs && tsdown",
     "typecheck": "tsc --noEmit",
-    "test": "vitest run",
+    "test": "pnpm run build:css && vitest run",
     "build:css": "node scripts/build-client-css.mjs"
   },
   "peerDependencies": {