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/README.md

@@ -0,0 +1,43 @@
+# 3x-ui frontend
+
+Vue 3 + Ant Design Vue 4 + Vite. Builds into `../web/dist/`, which the
+Go binary will embed via `embed.FS` once the migration reaches the page
+handlers (Phase 4+).
+
+This directory exists alongside the legacy `web/html/` Vue 2 templates
+during the migration. Pages will move over one at a time on the
+`vue3-migration` branch.
+
+## Dev
+
+```sh
+cd frontend
+npm install
+npm run dev
+```
+
+The dev server runs on `http://localhost:5173/` and proxies API calls to
+the Go panel at `http://localhost:2053/` — start the Go panel first
+(`go run main.go`), then start Vite.
+
+## Production build
+
+```sh
+npm run build
+```
+
+Outputs to `../web/dist/`. The Go binary picks it up at compile time via
+`embed.FS`.
+
+## Where things live
+
+- `src/main.js` — app entrypoint (createApp, install Antd, mount)
+- `src/App.vue` — root component (currently a smoke-test placeholder)
+- `vite.config.js` — build + dev-server config
+- `index.html` — Vite HTML template
+
+## Adding new pages
+
+For each legacy page being migrated, add an entry to
+`vite.config.js` `rollupOptions.input`. Each entry produces its own
+HTML file in `web/dist/`, which the Go panel route handler will serve.