feat(cli): support configurable local state directory

Add --state-dir and ET_STATE_DIR for persisted web-client machine IDs.

Fixes #2340

easytier/locales/app.yml

@@ -13,6 +13,9 @@ core_clap:
       the machine id to identify this machine, used for config recovery after disconnection, must be unique and fixed. by default it is loaded from persisted local state; on first start it may be migrated from system information or generated, then remains fixed.
     zh-CN: |+
       Web 配置服务器通过 machine id 来识别机器，用于断线重连后的配置恢复，需要保证唯一且固定不变。默认从本地持久化状态读取；首次启动时可能基于系统信息迁移或生成，之后保持固定不变。
+  state_dir:
+    en: "directory for persistent local state, currently the Web config server machine ID. Uses the platform default when unset; ignored when --machine-id is specified."
+    zh-CN: "本地持久化状态目录，目前用于保存 Web 配置服务器的机器 ID。未指定时使用平台默认目录；指定 --machine-id 时不使用此目录。"
   config_file:
     en: "path to the config file, NOTE: the options set by cmdline args will override options in config file"
     zh-CN: "配置文件路径，注意：命令行中的配置的选项会覆盖配置文件中的选项"

easytier/src/core.rs

@@ -100,6 +100,13 @@ struct Cli {
     )]
     machine_id: Option<String>,
 
+    #[arg(
+        long,
+        env = "ET_STATE_DIR",
+        help = t!("core_clap.state_dir").to_string()
+    )]
+    state_dir: Option<PathBuf>,
+
     #[arg(
         short,
         long,
@@ -1536,7 +1543,7 @@ async fn run_main(cli: Cli) -> anyhow::Result<()> {
             config_server_url_s,
             crate::common::MachineIdOptions {
                 explicit_machine_id: cli.machine_id.clone(),
-                state_dir: None,
+                state_dir: cli.state_dir.clone(),
             },
             cli.network_options.hostname.clone(),
             cli.network_options.secure_mode.unwrap_or(false),