fix: clarify routing threshold hint about default value

- Update routing_threshold_hint (en/zh) to note that values <= 0 use
  the system default (0.35)

web/frontend/src/i18n/locales/en.json

@@ -528,7 +528,7 @@
       "routing_light_model": "Light Model",
       "routing_light_model_hint": "Model name (from your model list) to use for simple tasks.",
       "routing_threshold": "Complexity Threshold",
-      "routing_threshold_hint": "Complexity score threshold (0–1). Requests scoring above this use the primary model.",
+      "routing_threshold_hint": "Complexity score threshold (0–1). Values \u2264 0 use the default (0.35). Requests scoring above this use the primary model.",
       "filter_sensitive_data": "Filter Sensitive Data",
       "filter_sensitive_data_hint": "Automatically filter API keys, tokens, and secrets from tool results before sending to the LLM.",
       "filter_min_length": "Filter Min Content Length",

web/frontend/src/i18n/locales/zh.json

@@ -528,7 +528,7 @@
       "routing_light_model": "轻量模型",
       "routing_light_model_hint": "用于简单任务的模型名称（来自模型列表）。",
       "routing_threshold": "复杂度阈值",
-      "routing_threshold_hint": "复杂度评分阈值（0–1）。评分高于此值的请求使用主模型。",
+      "routing_threshold_hint": "复杂度评分阈值（0–1]。留空或设为 0 使用系统默认值（当前为 0.35）。评分高于此值的请求使用主模型。",
       "filter_sensitive_data": "过滤敏感数据",
       "filter_sensitive_data_hint": "自动过滤工具返回结果中的 API 密钥、令牌和机密信息，避免发送给 LLM。",
       "filter_min_length": "过滤最小内容长度",