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

backend/internal/model/model.go

@@ -41,6 +41,7 @@ type Instance struct {
 	AccessMode    AccessMode `json:"access_mode"`   // open | whitelist
 	AllowNetworks []string   `json:"allow_networks"` // 实例级白名单 CIDR 列表
 	AuthMode      AuthMode   `json:"auth_mode"`     // cert | cert+password
+	PublicHost    string     `json:"public_host"`    // 公网域名/IP,生成 .ovpn 时优先用它;空则用下载时前端传入的 host
 	Status        string     `json:"status"`        // running/stopped/error
 	PID           int        `json:"pid"`
 	CreatedAt     time.Time  `json:"created_at"`