dsh-download-indicator: Add a floating download progress indicator to DSH Web UI

The DSH plugin maintained by falleneve-ctrl provides a semi-transparent, draggable download progress floating window for the Web UI. It monitors a specified directory, polling every 2 seconds, and appears only while the file is still growing. It displays a progress bar, the amount downloaded, and the measured speed, automatically fading out when idle. The window supports drag-and-drop positioning and saves its state to localStorage. If the total file size is unknown, it displays an indeterminate progress bar rather than fake percentages. By default, it monitors `F:\ComfyUI\models` and `F:\training\downloads`, which can be overridden via the `DSH_DOWNLOAD_WATCH` environment variable. The server registers routes for `status` and `widget.js`, deliberately utilizing inline script injection to prevent loading failures that would prevent the client from starting.