feat: add traffic abuse risk assessment with color gradation

Add threshold inputs (total GB/day, per-node GB/day) to admin traffic
usage page. When set, rows get background color gradation (green to red)
and node cells get text color (blue to red) based on threshold ratios.
Conditional Risk column shows composite risk badge with level indicator.
7 i18n keys added across all 4 locales (ru, en, zh, fa).

src/locales/zh.json

@@ -696,7 +696,14 @@
       "statusDisabled": "已禁用",
       "customDates": "自定义日期",
       "dateFrom": "从",
-      "dateTo": "到"
+      "dateTo": "到",
+      "totalThreshold": "总流量GB/天",
+      "nodeThreshold": "节点GB/天",
+      "risk": "风险",
+      "riskLow": "低",
+      "riskMedium": "中",
+      "riskHigh": "高",
+      "riskCritical": "严重"
     },
     "paymentMethods": {
       "title": "支付方法",