fix(fc2c-i): vitest needs plugin-vue to transform SFCs in router.spec

frontend/vitest.config.js

@@ -1,6 +1,11 @@
 import { defineConfig } from 'vitest/config'
+import vue from '@vitejs/plugin-vue'
 
+// plugin-vue is required so specs that import router.js (which imports .vue
+// SFCs) can transform those single-file components. No component is mounted
+// in the suite (router.resolve / pure-logic only), so jsdom is unnecessary.
 export default defineConfig({
+  plugins: [vue()],
   test: {
     environment: 'node',
     include: ['test/**/*.spec.js'],