build(frontend): Phase 2 — scaffold Vite + Vue 3 + AD-Vue 4

Adds a frontend/ directory that lives alongside the legacy web/html/
Vue 2 templates during the migration. Vite builds into ../web/dist/
so the Go binary will be able to embed the result via embed.FS once
Phase 4 starts moving real pages over.

- package.json pins Vue 3.5, Ant Design Vue 4.2, Vite 6, vue-i18n 10
- vite.config.js: dev server on :5173 with API proxy to the Go panel
  on :2053; build output to ../web/dist/
- src/App.vue is currently a smoke-test placeholder — delete once the
  first real page (login) lands in Phase 4
- node_modules and dist are already ignored at repo root

To verify locally:
  cd frontend && npm install && npm run dev

Pages will be migrated one at a time on the vue3-migration branch.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/.gitignore

@@ -0,0 +1,3 @@
+node_modules/
+.vite/
+*.log