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/en.js

@@ -624,4 +624,7 @@ export default {
   "routing.tasks.addModel": "model",
   "routing.bands.enable": "My complexity boundaries",
   "routing.bands.hint": "Drag the handles to set where low ends and where high begins — the handles push each other, the numbers update live. The router's continuous score is re-mapped with YOUR boundaries; unchecked = the router's own bands.",
+  "tier.low": "low",
+  "tier.medium": "medium",
+  "tier.high": "high",
 };