dsh-plugin-file-attach: Add Local Files to the DSH Context

The DSH Web plugin addresses local file injection into the model context through a UI entry point. Users can add files via a button, with the content automatically injected into the context without manual copying. The plugin supports text, code, Markdown, JSON, and images, with a total size limit of 1MiB. Files are stored only in the current session, must be re-added after a refresh, and do not pollute the chat history.