feat: add DHCP subnet (CIDR) configuration parameter

Allow -d to optionally accept a CIDR subnet (e.g. -d 10.0.0.0/24)
to fix the DHCP address range. When specified, the DHCP allocator
uses the configured subnet instead of deriving it from peer IPs.

Changes:
- Add dhcp_cidr field to config struct and ConfigLoader trait
- Modify -d CLI arg to accept optional CIDR string value
- Update DHCP logic to use configured CIDR as default subnet
- Add dhcp_cidr to NetworkConfig proto message
- Update launcher to handle dhcp_cidr in gen_config/new_from_config
- Update i18n translations

easytier/src/common/config.rs

@@ -185,6 +185,9 @@ pub trait ConfigLoader: Send + Sync {
     fn get_dhcp(&self) -> bool;
     fn set_dhcp(&self, dhcp: bool);
 
+    fn get_dhcp_cidr(&self) -> Option<cidr::Ipv4Cidr>;
+    fn set_dhcp_cidr(&self, cidr: Option<cidr::Ipv4Cidr>);
+
     fn add_proxy_cidr(
         &self,
         cidr: cidr::Ipv4Cidr,
@@ -535,6 +538,7 @@ struct Config {
     ipv6_public_addr_auto: Option<bool>,
     ipv6_public_addr_prefix: Option<String>,
     dhcp: Option<bool>,
+    dhcp_cidr: Option<String>,
     network_identity: Option<NetworkIdentity>,
     listeners: Option<Vec<url::Url>>,
     mapped_listeners: Option<Vec<url::Url>>,
@@ -768,6 +772,18 @@ impl ConfigLoader for TomlConfigLoader {
         self.config.lock().unwrap().dhcp = Some(dhcp);
     }
 
+    fn get_dhcp_cidr(&self) -> Option<cidr::Ipv4Cidr> {
+        let locked_config = self.config.lock().unwrap();
+        locked_config
+            .dhcp_cidr
+            .as_ref()
+            .and_then(|s| s.parse().ok())
+    }
+
+    fn set_dhcp_cidr(&self, cidr: Option<cidr::Ipv4Cidr>) {
+        self.config.lock().unwrap().dhcp_cidr = cidr.map(|c| c.to_string());
+    }
+
     fn add_proxy_cidr(
         &self,
         cidr: cidr::Ipv4Cidr,