DSH Preview Plugin: Embed a Development Server in an iframe and Transparently Proxy SPA Traffic

`dsh-preview-plugin` is a plugin designed for DeepSeek Harness (DSH) to address development preview issues in SPA applications. It embeds via an iframe and transparently proxies the server, supporting single-page routing, static asset loading, and multi-port backend proxying. Users need to add the dependency and bundle configuration in `package.json`, and enter the frontend port and an optional backend port in the Preview tab of the DSH Web GUI to start the preview. The plugin supports SPA routing, form submission, cookie pass-through, and path persistence, but native browser navigation requires the application to build URLs using relative paths internally.