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/en.js

@@ -622,4 +622,6 @@ export default {
   "routing.tasks.tierHint": "Which model serves this task at which complexity; \"any\" leads at every tier. Order = fallback order. High verdicts still escalate over any-tier pins.",
   "routing.tasks.tierAny": "any complexity",
   "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.",
 };