feat: domain validation for NTP servers and mail domain fields

- isValidDomain / isValidDomainOrIp helpers (RFC-compliant label regex)
- network.ntp_servers: each entry validated as hostname or IP; invalid entry shown in error message
- email.domain: validated as a proper domain name; blocks autosave until fixed

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

webui/src/pages/Settings.jsx

@@ -137,9 +137,22 @@ function isValidIpCidr(v) {
   return [a, b, c, d].every(n => n >= 0 && n <= 255) && p >= 0 && p <= 32;
 }
 
+function isValidDomain(v) {
+  if (!v || !v.trim()) return false;
+  const s = v.trim();
+  if (s.length > 253) return false;
+  return /^[a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(\.[a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/.test(s);
+}
+
+function isValidDomainOrIp(v) {
+  const s = (v || '').trim();
+  return isValidIp(s) || isValidDomain(s);
+}
+
 const E_PORT   = 'Must be 1–65535';
 const E_IP     = 'Must be a valid IP address';
 const E_CIDR   = 'Must be a valid IP/CIDR (e.g. 10.0.0.1/24)';
+const E_DOMAIN = 'Must be a valid domain (e.g. mail.example.com)';
 
 function validateServiceConfig(key, data) {
   const errors = {};
@@ -156,6 +169,8 @@ function validateServiceConfig(key, data) {
       else if (parts[1]?.trim() && !isValidIp(parts[1].trim()))
         errors.dhcp_range = `End IP is invalid`;
     }
+    const badNtp = (data.ntp_servers || []).find(s => !isValidDomainOrIp(s));
+    if (badNtp !== undefined) errors.ntp_servers = `"${badNtp}" is not a valid hostname or IP`;
   }
   if (key === 'wireguard') {
     port('port');
@@ -163,6 +178,7 @@ function validateServiceConfig(key, data) {
   }
   if (key === 'email') {
     port('smtp_port'); port('submission_port'); port('imap_port'); port('webmail_port');
+    if (data.domain && !isValidDomain(data.domain)) errors.domain = E_DOMAIN;
   }
   if (key === 'calendar') port('port');
   if (key === 'files') { port('port'); port('manager_port'); }
@@ -291,7 +307,7 @@ function NetworkForm({ data, onChange, errors = {} }) {
       <Field label="DHCP Range" hint="e.g. 10.0.0.100,10.0.0.200,12h" error={errors.dhcp_range}>
         <TextInput value={data.dhcp_range} onChange={(v) => onChange({ ...data, dhcp_range: v })} placeholder="10.0.0.100,10.0.0.200,12h" />
       </Field>
-      <Field label="NTP Servers">
+      <Field label="NTP Servers" hint="Hostnames or IPs" error={errors.ntp_servers}>
         <TagList value={data.ntp_servers || []} onChange={(v) => onChange({ ...data, ntp_servers: v })} placeholder="0.pool.ntp.org" />
       </Field>
     </div>
@@ -317,7 +333,7 @@ function WireguardForm({ data, onChange, errors = {} }) {
 function EmailForm({ data, onChange, errors = {} }) {
   return (
     <div className="space-y-3">
-      <Field label="Mail Domain">
+      <Field label="Mail Domain" error={errors.domain}>
         <TextInput value={data.domain} onChange={(v) => onChange({ ...data, domain: v })} placeholder="mail.example.com" />
       </Field>
       <Field label="SMTP Port" hint="MTA-to-MTA (default 25)" error={errors.smtp_port}>