feat: add language selection to system settings

Add a language selector to the System Settings page supporting English,
Japanese, and Chinese. The i18n dictionary already contained zh
translations but lacked the UI to switch languages.

Changes:
- html/i18n.js: add sys_language key to en/ja/zh
- html/system.html: add language selector dropdown
- html/system.js: add changeLang() and selector initialization
- tools/httpd_sim.c: fix cookie parsing for multi-cookie headers;
  fix Set-Cookie response (missing Path=/, missing \r\n\r\n);
  add SO_REUSEADDR for faster port reuse

html/system.js

@@ -2,6 +2,11 @@ var systemInterval = Number();
 var isSaving = false;
 const ips = ["ip", "netmask", "gw"];
 
+function changeLang() {
+  var lang = document.getElementById('lang-select').value;
+  setLang(lang);
+}
+
 function checkIp(ip) {
   const ipv4 = /^(\d{1,3}\.){3}\d{1,3}$/;
   if (!ipv4.test(ip)) {alert(t('sys_invalid_ip') + ip); return false };
@@ -146,5 +151,7 @@ function resetSwitch() {
 }
 
 window.addEventListener("load", function() {
+  var langSel = document.getElementById('lang-select');
+  if (langSel) langSel.value = rtlLang;
   systemInterval = setInterval(fetchIP, 1000);
 });