Fix segment fetch URLs and add multi-stream zoom column layout
- Use directory-level stream name in cluster_segments() instead of
fragile stream.json marker value
- Include stream path component in JS fetch URLs for segment
content load and delete (fixes 404s)
- Render zoom timeline pills in side-by-side columns when multiple
streams exist on a day
- Add import.apple fixture stream on 20240101 for multi-stream testing