feat: 新增 IPv6 双栈支持

- 协议: InitMessage 增加 ip6/prefix6/server_ip6 字段(omitempty,向后兼容)
- TUN: Device 接口新增 ConfigureIPv6, macOS 用 ifconfig inet6, Linux 用 ip -6 addr add
- 路由: full 模式自动添加 ::/1+8000::/1, v6 服务器旁路 /128, resolveHosts 双栈解析
- 会话: setupTUN 解析 v6 字段并配置 TUN+路由, connectOnce 传 v6 给 stats/日志
- 统计: SetConnected(ip,ip6), Snapshot.AssignedIP6
- DB: connection_logs 增加 assigned_ip6 列 + migrateV3 幂等迁移
- UI: 状态卡片新增独立 IPv6 行, 始终可见(未获取显示 IPv6: —)
- 文档: 同步 client-development.md 至服务端 IPv6 版本

internal/tun/tun.go

@@ -18,6 +18,10 @@ type Device interface {
 	Write(p []byte) (int, error)
 	// Configure sets the interface address, prefix, and peer IP.
 	Configure(localIP net.IP, prefix int, peerIP net.IP) error
+	// ConfigureIPv6 sets a secondary IPv6 address and prefix on the
+	// interface. Unlike Configure, there is no peer IP because macOS
+	// utun IPv6 does not use the point-to-point aliasing form.
+	ConfigureIPv6(localIP6 net.IP, prefix6 int) error
 	// SetMTU sets the interface MTU.
 	SetMTU(mtu int) error
 	// Close destroys the TUN device.