dsh-browser-fs: Enable the dsh agent to read and write local files on the browser's machine

dsh-browser-fs is a dsh client plugin designed to address the issue where remotely deployed dsh agents cannot directly access the user's local browser files. This plugin utilizes the File System Access API, allowing users to authorize local directories and forward tool calls to the browser for execution via a WebSocket channel. Core features include registering three model tools, `browser_fs_list`, `browser_fs_read`, and `browser_fs_write`, which support directory listing, UTF-8 text reading (default limit of 256 KiB), and writing. The browser interface provides a floating card to display connection and authorization status, supporting directory tree browsing, file search, and image/text preview (text preview limit 64K).