Add ESLint to the frontend

Wires up @nuxt/eslint (the official Nuxt module) so `npm run lint`,
already documented in CLAUDE.md's pre-completion checklist, actually
exists. It auto-generates a flat config aware of Nuxt's auto-imports,
so it doesn't flag composables and utils that Nuxt injects without an
explicit import.

frontend/nuxt.config.ts

@@ -33,4 +33,6 @@ export default defineNuxtConfig({
   typescript: {
     strict: true,
   },
+
+  modules: ['@nuxt/eslint'],
 })