feat: Linear-style UI redesign with improved mobile experience

Major changes:
- Redesign cabinet with Linear-style components and top navigation
- Replace detail modals with dedicated pages (users, broadcasts, email templates)
- Add email channel support for broadcasts
- Remove pull-to-refresh, improve drag-and-drop on touch devices
- Fix Telegram Mini App: fullscreen, back button, scroll restoration
- Unify admin pages color scheme to design system
- Mobile-first improvements: horizontal tabs for settings, better touch targets

src/locales/zh.json

@@ -25,6 +25,7 @@
     "refresh": "刷新",
     "retry": "重试",
     "saving": "保存中...",
+    "understand": "明白了",
     "units": {
       "gb": "GB",
       "perGb": "/GB"
@@ -705,7 +706,9 @@
       "resetted": "模板已恢复默认",
       "testSent": "测试邮件已发送",
       "variables": "可用变量",
-      "clickToCopy": "点击复制"
+      "clickToCopy": "点击复制",
+      "previewDesktopOnly": "预览仅在桌面网页版可用",
+      "previewNotAvailable": "预览不可用"
     },
     "payments": {
       "title": "支付验证",