feat: 添加设置入口并统一背景样式

新增设置菜单入口，将其放在侧边栏底部系统信息区下方
统一背景绘制到 html 元素上，避免元素高度差导致的留白
临时下架背景选项 bg-3，调整侧边栏布局与样式
新增中英文的设置翻译文本，优化系统信息区的间距

src/i18n/locales/en-US.json

@@ -22,7 +22,8 @@
     "dashboard": "Dashboard",
     "servers": "Servers",
     "tunnels": "Tunnels",
-    "download": "Download frpc"
+    "download": "Download frpc",
+    "setting": "Settings"
   },
   "system": {
     "cpu": "CPU usage",

src/i18n/locales/zh-CN.json

@@ -22,7 +22,8 @@
     "dashboard": "仪表盘",
     "servers": "服务器",
     "tunnels": "隧道列表",
-    "download": "Frpc 下载"
+    "download": "Frpc 下载",
+    "setting": "设置"
   },
   "system": {
     "cpu": "CPU 占用",