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.js

@@ -48,6 +48,11 @@ export function onLangChange(fn) {
   return () => listeners.delete(fn);
 }
 
+// Human name for a complexity tier; custom tier names pass through as-is.
+export function trTier(tier) {
+  return ["low", "medium", "high"].includes(tier) ? t("tier." + tier) : tier;
+}
+
 export function t(key, vars) {
   const d = DICTS[lang] || en;
   let s = d[key] ?? en[key] ?? key;