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/src/App.vue

@@ -0,0 +1,48 @@
+<script setup>
+import { ref } from 'vue';
+
+const message = ref('Vue 3 + Ant Design Vue 4 scaffold is alive');
+const count = ref(0);
+</script>
+
+<template>
+  <a-layout class="layout">
+    <a-layout-header class="header">
+      <h1>3x-ui (vue3-migration scaffold)</h1>
+    </a-layout-header>
+    <a-layout-content class="content">
+      <a-space direction="vertical" :size="16" style="width: 100%">
+        <a-alert :message="message" type="success" show-icon />
+        <a-card title="Smoke test">
+          <p>If you see this card with a styled button below, the toolchain works.</p>
+          <a-space>
+            <a-button type="primary" @click="count++">Clicked {{ count }} times</a-button>
+            <a-button @click="count = 0">Reset</a-button>
+          </a-space>
+        </a-card>
+      </a-space>
+    </a-layout-content>
+  </a-layout>
+</template>
+
+<style>
+.layout {
+  min-height: 100vh;
+}
+.header {
+  background: #001529;
+  color: #fff;
+  display: flex;
+  align-items: center;
+  padding: 0 24px;
+}
+.header h1 {
+  color: #fff;
+  margin: 0;
+  font-size: 18px;
+}
+.content {
+  padding: 24px;
+  background: #f0f2f5;
+}
+</style>

frontend/src/main.js

@@ -0,0 +1,9 @@
+import { createApp } from 'vue';
+import Antd from 'ant-design-vue';
+import 'ant-design-vue/dist/reset.css';
+
+import App from './App.vue';
+
+const app = createApp(App);
+app.use(Antd);
+app.mount('#app');