feat(image-popup): add auto-popup for detected screenshots and images

Implements image file watching in session working directories with
automatic popup display in the web UI:

- Add ImageWatcher class using chokidar for cross-platform file watching
- Add ImageDetectedEvent type for SSE broadcasting
- Integrate with session lifecycle (watch on create, unwatch on delete)
- Add image:detected SSE event handler in frontend
- Create draggable image popup window with open-in-new-tab support
- Add CSS styling with compact wizard layout

Supports: png, jpg, jpeg, gif, webp, bmp, svg

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -42,6 +42,7 @@
     "@fastify/static": "^8.0.0",
     "@modelcontextprotocol/sdk": "^1.25.3",
     "chalk": "^5.3.0",
+    "chokidar": "^3.6.0",
     "commander": "^12.1.0",
     "fastify": "^5.1.0",
     "ink": "^6.6.0",