fix(model-downloader): subscribe to progress via the ComfyUI API client - #120

Follow-up from #119.

Problem

model_downloader.js registered its model_download_progress listener only through window.api, which no current frontend defines (they expose window.comfyAPI.api.api). Those registrations were no-ops, so:

• Progress arrived only through the WebSocket onmessage monkey-patch fallback.
• Frontend 1.57 logs console.error("Unknown message type model_download_progress") for any message type without a listener. 1.52 logged the same thing as a warning.

Fix

• Resolve the API client as window.comfyAPI?.api?.api || window.app?.api || window.api and call addEventListener('model_download_progress', …) on it.
• The core's handleMessageEvent already unwraps CustomEvent.detail.
• The WebSocket patch is kept only as a fallback when no API client is found.
• The dead registerExtension, registerMessageHandler and reportedUnknownMessageTypes paths are removed.

Testing

Headless Chromium (Playwright) against nix run .#default on ./data with ComfyUI 0.44.0 and frontend 1.57.6:

• Unknown message type model_download_progress no longer appears in the console.
• The "Successfully patched WebSocket handler" log is gone, so the listener path is the one in use.
• A detached <a download> click was intercepted, and the backend download completed at 100%. Two progress events reached handleMessageEvent.
• The only remaining console errors are the Impact-Pack clipspace.js 404s, which already existed before this change.