feat: 实现第三层 TUN VPN 服务，支持后台 IP 分配

- 新增 TUN 设备层（water 库），分 linux/darwin 平台配置 IP/路由/MTU
- 实现 IP 分配管理：动态池自动分配 + 按用户静态预留，支持热更新
- 实现 PacketSwitch 共享 TUN 包转发：源 IP 防伪、按目的 IP 查表转发、allow-c2c
- 重写隧道：自研简化 WS 协议（文本帧 JSON 控制 init/ready，二进制帧=原始 IP 包）
- VpnService 单例管理 TUN 生命周期，子网变更踢线重建，预留增删热更新
- 新增 vpn_settings/vpn_reservations 表，AutoMigrate + 默认设置 seed
- 新增 Admin API：settings 读写、status、clients、reservations CRUD
- 前端新增 VpnView（/admin/vpn）：状态面板/设置表单/在线客户端/静态预留
- main.go 启动时按 DB 设置初始化 VPN 服务

go.mod

@@ -3,8 +3,17 @@ module lmvpn
 go 1.26.4
 
 require (
+	github.com/apparentlymart/go-cidr v1.1.1
 	github.com/gin-gonic/gin v1.12.0
+	github.com/glebarez/sqlite v1.11.0
 	github.com/goccy/go-yaml v1.19.2
+	github.com/golang-jwt/jwt/v5 v5.3.1
+	github.com/google/uuid v1.3.0
+	github.com/gorilla/websocket v1.5.3
+	github.com/songgao/water v0.0.0-20200317203138-2b4b6d7c09d8
+	golang.org/x/crypto v0.53.0
+	gorm.io/driver/mysql v1.6.0
+	gorm.io/gorm v1.31.2
 )
 
 require (
@@ -17,15 +26,11 @@ require (
 	github.com/gabriel-vasile/mimetype v1.4.12 // indirect
 	github.com/gin-contrib/sse v1.1.0 // indirect
 	github.com/glebarez/go-sqlite v1.21.2 // indirect
-	github.com/glebarez/sqlite v1.11.0 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
 	github.com/go-playground/validator/v10 v10.30.1 // indirect
 	github.com/go-sql-driver/mysql v1.8.1 // indirect
 	github.com/goccy/go-json v0.10.5 // indirect
-	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
-	github.com/google/uuid v1.3.0 // indirect
-	github.com/gorilla/websocket v1.5.3 // indirect
 	github.com/jinzhu/inflection v1.0.0 // indirect
 	github.com/jinzhu/now v1.1.5 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
@@ -42,13 +47,10 @@ require (
 	github.com/ugorji/go/codec v1.3.1 // indirect
 	go.mongodb.org/mongo-driver/v2 v2.5.0 // indirect
 	golang.org/x/arch v0.22.0 // indirect
-	golang.org/x/crypto v0.53.0 // indirect
 	golang.org/x/net v0.55.0 // indirect
 	golang.org/x/sys v0.46.0 // indirect
 	golang.org/x/text v0.38.0 // indirect
 	google.golang.org/protobuf v1.36.10 // indirect
-	gorm.io/driver/mysql v1.6.0 // indirect
-	gorm.io/gorm v1.31.2 // indirect
 	modernc.org/libc v1.22.5 // indirect
 	modernc.org/mathutil v1.5.0 // indirect
 	modernc.org/memory v1.5.0 // indirect