Add interactive tooltips and hover highlights to all charts

[islubee]

Summary

• Adds a Tooltip component — absolute-positioned HTML overlay that renders near the cursor
• All five charts (BarChart, Histogram, ScatterPlot, Timeline, PieChart) show contextual tooltips on hover
• CSS :has() rules dim non-hovered siblings (bars, dots, pie slices) to 35% opacity with a 150ms transition
• Timeline uses a transparent SVG overlay rect + d3.bisect to snap to the nearest data point, drawing a dashed hairline and dot indicator
• Bars and Circles components updated to accept onMouseEnter(d, i, e) / onMouseLeave / onMouseMove callbacks

Tooltip content per chart

Chart	Tooltip shows
BarChart	category + value
Histogram	bin range + count
ScatterPlot	x value + y value
Timeline	formatted date + y value
PieChart	label + value + % of total

Test plan

• Run npm test — all 72 tests pass
• Hover over bars in BarChart — tooltip appears, non-hovered bars dim
• Hover over bars in Histogram — tooltip shows bin range and count
• Hover over dots in ScatterPlot — tooltip appears, other dots dim
• Move mouse over Timeline — hairline snaps to nearest data point, tooltip updates
• Hover over pie slices — tooltip shows label, value, and percentage; other slices dim
• Tooltip stays within chart bounds on edge data points