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:',