feat: 每用户最大连接数改为可配置，默认 30

将 maxConnsPerUser 从硬编码常量(3)改为数据库动态配置项，管理员可在
/admin/vpn 隧道设置中调整，保存后对新连接立即生效。

- model/vpn.go: VpnSetting 新增 MaxConnsPerUser 字段，gorm default:30
- db/db.go: 种子数据设默认 30；旧库回填 0 值为 30
- vpn/tunnel.go: 删除 maxConnsPerUser 常量，改读 VPN.Settings()，兜底 30
- handler/vpn.go: API 响应/请求结构体新增字段，校验范围 1-1000
- VpnView.vue: 隧道设置表单新增"每用户最大连接数"输入框
- zh.ts/en.ts: 新增 maxConnsPerUser 文案，更新首页多设备描述
- docs/client-development.md: 更新常量表为可配置项

frontend/src/locales/en.ts

@@ -133,6 +133,7 @@ export default {
     serverConfigTunIp: 'Server Configures TUN IP',
     autoConfig: 'Auto',
     manual: 'Manual',
+    maxConnsPerUser: 'Max Connections Per User',
     saveSettings: 'Save Settings',
     saveSuccess: 'Saved successfully',
     user: 'User',
@@ -168,7 +169,7 @@ export default {
       'Admin and user roles with full user CRUD, enable/disable, and password changes, plus self-protection rules.',
     multiDevice: 'Multi-Device Concurrent',
     multiDeviceDesc:
-      'Up to 3 concurrent connections per user with automatic tunnel IP assignment.',
+      'Supports multi-device concurrent connections per user, configurable in admin panel.',
     dualStack: 'IPv4/IPv6 Dual-Stack',
     dualStackDesc:
       'Supports both IPv4 and IPv6 subnets with NAT dual-stack forwarding and anti-spoofing.',

frontend/src/locales/zh.ts

@@ -132,6 +132,7 @@ export default {
     serverConfigTunIp: '服务端配置 TUN IP',
     autoConfig: '自动配置',
     manual: '手动',
+    maxConnsPerUser: '每用户最大连接数',
     saveSettings: '保存设置',
     saveSuccess: '保存成功',
     user: '用户',
@@ -165,7 +166,7 @@ export default {
     userManageDesc:
       '管理员与普通角色，支持用户增删改、启用禁用与改密，内置自保护规则。',
     multiDevice: '多设备并发',
-    multiDeviceDesc: '每用户最多 3 个并发连接，自动分配隧道 IP 地址。',
+    multiDeviceDesc: '每用户支持多设备并发连接，可在管理后台配置上限。',
     dualStack: 'IPv4/IPv6 双栈',
     dualStackDesc: '同时支持 IPv4 与 IPv6 子网，NAT 双栈转发与源地址反欺骗。',
     reservation: '静态 IP 预留',

frontend/src/views/VpnView.vue

@@ -16,6 +16,7 @@ interface Settings {
   allow_client_to_client: boolean
   do_local_ip_config: boolean
   do_remote_ip_config: boolean
+  max_conns_per_user: number
 }
 interface ClientInfo {
   user_id: number
@@ -83,6 +84,7 @@ const form = ref<Settings>({
   allow_client_to_client: false,
   do_local_ip_config: true,
   do_remote_ip_config: true,
+  max_conns_per_user: 30,
 })
 
 async function fetchSettings() {
@@ -403,6 +405,10 @@ onMounted(() => {
             <option :value="false">{{ t('vpn.manual') }}</option>
           </select>
         </div>
+        <div>
+          <label class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-1">{{ t('vpn.maxConnsPerUser') }}</label>
+          <input v-model.number="form.max_conns_per_user" type="number" min="1" max="1000" class="w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-gray-700 text-gray-900 dark:text-white" />
+        </div>
       </div>
       <div class="flex items-center gap-4 mt-6">
         <button