feat(routing): linked complexity-boundary sliders — your own low/medium/high

The tier boundaries lived on the router and were global for every customer
of a hosted router; per-gateway tuning meant hand-editing score ranges. The
Routing page now has one track with two draggable handles (the low|medium
and medium|high boundaries) over three colored zones: handles push each
other (low can't cross medium — it shoves it along, and vice versa), the
numbers ride on the handles and the legend updates live, so nobody types
ranges by hand. Unchecked = the router's own bands, untouched behavior.

Under the hood the gateway re-maps the router's CONTINUOUS complexity score
with the user's boundaries ([routing.bands] low/medium in gateway.toml,
validated 0 < low < medium < 1) before tier chains, escalation and stats —
verified live: with bands at 0.70/0.72 a score-0.27 request that used to be
medium routed low, and back after restoring the defaults.

2 new strings in all 7 languages.

web/js/i18n/fr.js

@@ -629,4 +629,6 @@ export default {
   "routing.tasks.tierHint": "Quel modèle sert cette tâche à quelle complexité ; « toute » mène à tous les niveaux. L'ordre = ordre de repli. Un verdict high escalade toujours au-dessus des entrées « toute ».",
   "routing.tasks.tierAny": "toute complexité",
   "routing.tasks.addModel": "modèle",
+  "routing.bands.enable": "Mes limites de complexité",
+  "routing.bands.hint": "Faites glisser les poignées : où finit low et où commence high — elles se poussent l'une l'autre, les chiffres se mettent à jour en direct. Le score continu du routeur est re-découpé avec VOS limites ; décoché = les bandes du routeur.",
 };