Replace @nivo/sankey with custom D3 implementation:
- 30px thin node bars with labels positioned beside them
- Links at 0.3 opacity, 0.6 on hover with full path highlighting
- Colors based on first URL segment for visual grouping
- Dynamic height based on tallest column
- Responsive width via ResizeObserver
- Click nodes to filter, hover for tooltips
- Invisible wider hit areas for easier link hovering
- Remove @nivo/sankey dependency, add d3
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