fix(i18n): complexity tier names are translated everywhere they're shown

low/medium/high leaked as raw English into the bands legend, the tier badges
on the Routing page, the pair complexity selects, the benchmark's tier column
and the Playground decision panel. New trTier() helper renders the human name
(ru: лёгкий/средний/сложный, de: leicht/mittel/schwer, fr/es/zh/tr likewise);
custom tier names pass through untouched, badge colors stay keyed by the raw
tier. Verified in the browser in Russian and German.

web/js/i18n/zh.js

@@ -631,4 +631,7 @@ export default {
   "routing.tasks.addModel": "模型",
   "routing.bands.enable": "自定义复杂度边界",
   "routing.bands.hint": "拖动手柄设定 low 到哪里结束、high 从哪里开始 — 手柄会相互推动，数字实时更新。路由器的连续得分将按你的边界重新划分；取消勾选则使用路由器自身的边界。",
+  "tier.low": "简单",
+  "tier.medium": "中等",
+  "tier.high": "困难",
 };