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
- nodeThickness 8 → 100 (wide blocks like Rybbit)
- Labels inside nodes with white text instead of outside
- Margins 90px → 16px (labels no longer need outside space)
- Dynamic chart height based on max nodes per step
- Tighter nodeSpacing (4px) and subtle link opacity
- nodeBorderRadius 4 for rounded block corners
- Switch to fixed-width Sankey with horizontal scroll (250px per step)
- Thinner nodes (8px), tighter spacing (8px)
- Labels on all columns, not just first/last
- Lower link opacity (0.15) for cleaner look
- Increased node cap to 25 per step
