Add a scan progress indicator to the header that shows during library scans:
- Spinning SVG icon next to the BookHoard title
- Percentage display during active scans
- Dispatches bookhoard:scan-complete custom DOM event on window when scan
finishes, enabling other components (dashboard) to react without polling
- Auto-resets progress display after 3 seconds
- Uses WebSocket pub/sub via addListener/removeListener with cleanup on
header element removal