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/eslint.config.mjs

@@ -0,0 +1,6 @@
+// @ts-check
+import withNuxt from './.nuxt/eslint.config.mjs'
+
+export default withNuxt(
+  // Your custom configs here
+)