Feature: per-instance public_host for .ovpn

When creating/editing an instance, add '公网地址' (public_host) field.
This is the domain/IP clients will use to reach the OpenVPN server.
When downloading a .ovpn, this field is used first, falling back to:
  1. ?host= query parameter (one-time override)
  2. Request Host header
  3. 'vpn.example.com' default

This eliminates the need to manually edit .ovpn files after download
when the server's public address differs from its internal IP.

Backend changes:
  model.Instance: add PublicHost string field
  service.GenerateOVPN: priority PublicHost > remoteHost > default
  service.UpdateInstance: PublicHost persisted (already via JSON tag)

Frontend changes:
  Instances.vue: new '公网地址' input in create/edit dialog
  Instances.vue: new column in list table (shows '未配置' when empty)
  Users.vue: hint text updated to '可选: 覆盖实例公网地址'

E2E verified:
  public_host='vpn.yunwei.blog'  → .ovpn has 'remote vpn.yunwei.blog 1194'
  public_host='' + host=X        → .ovpn has 'remote X 1194'
  public_host set + no host      → public_host wins

frontend/src/views/Instances.vue

@@ -12,6 +12,12 @@
       <el-table-column prop="proto" label="协议" width="80" />
       <el-table-column prop="dev" label="设备" width="80" />
       <el-table-column prop="subnet" label="子网" />
+      <el-table-column prop="public_host" label="公网地址">
+        <template #default="{row}">
+          <span v-if="row.public_host" style="font-family:monospace">{{ row.public_host }}</span>
+          <span v-else class="muted">未配置</span>
+        </template>
+      </el-table-column>
       <el-table-column label="访问控制" width="110">
         <template #default="{row}">
           <el-tag :type="row.access_mode==='whitelist'?'warning':'info'" size="small">
@@ -53,6 +59,12 @@
           <el-select v-model="form.dev"><el-option label="tun" value="tun"/><el-option label="tap" value="tap"/></el-select>
         </el-form-item>
         <el-form-item label="子网"><el-input v-model="form.subnet" placeholder="10.8.0.0/24"/></el-form-item>
+        <el-form-item label="公网地址">
+          <el-input v-model="form.public_host" placeholder="vpn.example.com 或 203.0.113.10" />
+          <div class="muted" style="margin-top:4px">
+            客户端连接时用的服务端地址。<b>留空</b>则下载 .ovpn 时手动输入;<b>填写</b>则所有用户下载的 .ovpn 自动用此地址,无需修改。
+          </div>
+        </el-form-item>
         <el-form-item label="加密">
           <el-select v-model="form.cipher"><el-option label="AES-256-GCM" value="AES-256-GCM"/><el-option label="AES-128-GCM" value="AES-128-GCM"/><el-option label="CHACHA20-POLY1305" value="CHACHA20-POLY1305"/></el-select>
         </el-form-item>
@@ -103,7 +115,7 @@ const router = useRouter()
 
 async function load() { list.value = await Inst.list() }
 function openCreate() {
-  Object.assign(form, { name:'', port:1194, proto:'udp', dev:'tun', subnet:'10.8.0.0/24', cipher:'AES-256-GCM', auth_digest:'SHA256', push_dns:'', push_routes:'', extra:'', access_mode:'open', allow_networks:[], id:'' })
+  Object.assign(form, { name:'', port:1194, proto:'udp', dev:'tun', subnet:'10.8.0.0/24', cipher:'AES-256-GCM', auth_digest:'SHA256', push_dns:'', push_routes:'', extra:'', access_mode:'open', allow_networks:[], public_host:'', id:'' })
   dlg.value = true
 }
 function openEdit(row) {

frontend/src/views/Users.vue

@@ -7,7 +7,7 @@
           <el-option v-for="i in instances" :key="i.id" :value="i.id" :label="`${i.name} (${i.proto}/${i.port})`" />
         </el-select>
       </el-col>
-      <el-col :span="6"><el-input v-model="host" placeholder="客户端连接的远端域名/IP" /></el-col>
+      <el-col :span="6"><el-input v-model="host" placeholder="可选: 覆盖实例公网地址" /></el-col>
       <el-col :span="4"><el-button @click="loadUsers">刷新</el-button></el-col>
       <el-col :span="6" style="text-align:right">
         <el-button type="primary" :disabled="!instanceId" @click="openCreate"><el-icon><Plus /></el-icon>新建用户</el-button>