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/i18n.js

@@ -140,6 +140,7 @@ var LANG = {
     sys_ip: 'IP address:',
     sys_netmask: 'Netmask:',
     sys_gateway: 'Gateway:',
+    sys_language: 'Language:',
     sys_ip_note: 'When updating the above settings, remember to point your browser to the new IP afterwards:',
     sys_update: 'Update Settings',
     sys_save_label: 'Save all current settings to Flash:',
@@ -318,6 +319,7 @@ var LANG = {
     sys_ip: 'IP アドレス:',
     sys_netmask: 'ネットマスク:',
     sys_gateway: 'ゲートウェイ:',
+    sys_language: '言語:',
     sys_ip_note: '上記設定を変更した場合は、ブラウザで新しい IP にアクセスしてください:',
     sys_update: '設定更新',
     sys_save_label: '現在の設定をフラッシュに保存:',
@@ -496,6 +498,7 @@ var LANG = {
     sys_ip: 'IP 地址:',
     sys_netmask: '子网掩码:',
     sys_gateway: '网关:',
+    sys_language: '语言:',
     sys_ip_note: '更新上述设置后，请使用新的 IP 地址重新访问管理界面:',
     sys_update: '更新设置',
     sys_save_label: '将当前全部设置保存到 Flash:',

html/system.html

@@ -37,6 +37,16 @@
         <div class="lcol"> <label for="gw" data-i18n="sys_gateway">Gateway:</label></div>
         <div class="rcol"><input id="gw" class="ip" type="text" minlength="7" maxlength="15" size="15"/></div>
       </div>
+      <div class="row">
+        <div class="lcol"> <label data-i18n="sys_language">Language:</label></div>
+        <div class="rcol">
+          <select id="lang-select" onchange="changeLang()">
+            <option value="en">English</option>
+            <option value="ja">日本語</option>
+            <option value="zh">中文</option>
+          </select>
+        </div>
+      </div>
       <br/>
       <span data-i18n="sys_ip_note">When updating the above settings, remember to point your browser to the new IP afterwards:</span><br/>
       <input style="width:40%;" class="action" id="ip_sub" onclick="ipSub();" type="button" data-i18n="sys_update" value="Update Settings"><br/>

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);
 });