fsviewer: DeepSeek dsh Web Right-Side File Manager

`fsviewer` is a client plugin for DeepSeek Harness (DSH). Using the “everything is a plugin” architecture, it takes over the native right-side details pane and provides file management and preview features. The plugin supports directory trees, multi-file tabs, file source view switching, and workspace-isolated state management. Users only need to clone the source code and configure a profile to browse local files directly in a session. Note that it reads local files directly via API without authentication; **it is strictly prohibited in multi-user or network-exposed environments**.