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/zh.js

@@ -629,4 +629,6 @@ export default {
   "routing.tasks.tierHint": "哪个模型在哪个复杂度上处理该任务；「任意」在所有层级领先。顺序即回退顺序。high 判定仍会越过「任意」条目升级。",
   "routing.tasks.tierAny": "任意复杂度",
   "routing.tasks.addModel": "模型",
+  "routing.bands.enable": "自定义复杂度边界",
+  "routing.bands.hint": "拖动手柄设定 low 到哪里结束、high 从哪里开始 — 手柄会相互推动，数字实时更新。路由器的连续得分将按你的边界重新划分；取消勾选则使用路由器自身的边界。",
 };