system: configurable hostname (device identity)

Add a device hostname settable from the CLI (`hostname <text>`) and the
System Settings page. The value is sanitized on ingest to JSON-safe
printable ASCII (<=23 chars), stored in a shared __xdata buffer, seeded
to "RTLPlayground" at boot, persisted through the startup-config, and
reported in /information.json. It lives in the common header so other
modules can advertise it (LLDP uses it as the System Name TLV).

html/system.js

@@ -48,6 +48,14 @@ async function cmdSub() {
 }
 
 
+async function hostSub() {
+  const h = document.getElementById("hostname").value;
+  try { await fetch('/cmd', { method: 'POST', body: "hostname " + h }); }
+  catch(err) { console.error(`Error: ${err}`); }
+  fetchIP();
+}
+
+
 async function sendConfig(c) {
   if (isSaving) return;
   isSaving = true;
@@ -123,6 +131,7 @@ function fetchIP() {
       document.getElementById("ip").value=s.ip_address;
       document.getElementById("netmask").value=s.ip_netmask;
       document.getElementById("gw").value=s.ip_gateway;
+      if (document.getElementById("hostname")) document.getElementById("hostname").value = s.hostname || "";
       clearInterval(systemInterval);
       // Fetch and populate the config textbox
       fetchConfig().then((configText) => {