i18n: localize sidebar theme toggle, xray-status badge, and nodes menu

The sidebar theme submenu (Theme / Dark / Ultra dark) and the dashboard's
Xray status badge ("Xray is running" etc.) were hardcoded English strings.
Wire them through vue-i18n: ThemeSwitch.vue uses menu.theme/dark/ultraDark,
and XrayStatusCard.vue derives the badge text from the existing
pages.index.xrayStatus{Running,Stop,Error,Unknown} keys (status.js no
longer carries an English stateMsg field).

The "Nodes" menu item was already keyed as menu.nodes but only en-US and
fa-IR had a translation; add it to the other 11 languages, matching the
wording each file already uses for pages.nodes.title.
#4201

web/translation/zh-CN.json

@@ -94,6 +94,7 @@
     "ultraDark": "超暗色",
     "dashboard": "系统状态",
     "inbounds": "入站列表",
+    "nodes": "节点",
     "settings": "面板设置",
     "xray": "Xray 设置",
     "logout": "退出登录",