dsh-proxy-switch: Enable DSH's web_fetch to fetch web pages normally in a proxy environment

The DSH plugin maintained by lightrivers, with the package name @lightrivers/dsh-proxy-switch (v0.4.4, MIT). It automatically detects the local HTTP proxy, routing `web_fetch` through the proxy when active and automatically connecting directly when inactive. This addresses the failure of `web_fetch` reporting a "non-public IP address" caused by TUN fake-IP in Clash/Surge/v2rayN, as well as the issue where DSH fails to detect the proxy when only the system proxy is enabled. The default scope is limited to `web_fetch` and does not modify global network or process environment variables. This article covers the problems it solves, environment requirements, automatic detection and three routing modes, installation methods, panel usage, `state.json` configuration fields, the local status interface, working principle, and applicable notes.