feat(admin): show common placeholders group in email template editor

Companion to the bot-side change: the variables hint now renders two
groups — type-specific vars and common ones ({service_name},
{cabinet_url}, {support_username}, {username}, {email}, {date}) that
work in every template. The block is visible even for types with no
vars of their own (previously it was hidden entirely, looking like
placeholders didn't exist). Common chips insert at cursor the same way.

src/locales/zh.json

@@ -1461,7 +1461,8 @@
       "insertDefault": "插入默认模板",
       "insertDefaultConfirm": "用默认模板替换当前内容？",
       "testRecipientPlaceholder": "收件人邮箱（默认为您的邮箱）",
-      "testHint": "发送当前编辑器内容，变量将替换为示例值。"
+      "testHint": "发送当前编辑器内容，变量将替换为示例值。",
+      "variablesCommon": "通用变量（适用于所有模板）"
     },
     "payments": {
       "title": "支付",