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/es.js

@@ -631,4 +631,7 @@ export default {
   "routing.tasks.addModel": "modelo",
   "routing.bands.enable": "Mis límites de complejidad",
   "routing.bands.hint": "Arrastra los tiradores: dónde termina low y dónde empieza high — se empujan entre sí y las cifras se actualizan en vivo. La puntuación continua del router se re-mapea con TUS límites; sin marcar = las bandas del propio router.",
+  "tier.low": "fácil",
+  "tier.medium": "medio",
+  "tier.high": "difícil",
 };