Closes#2011.
time-series and sankey each created their cursor-following tooltip with a
duplicated className literal that had already drifted apart: time-series was
missing `text-primary` and `z-50`. Move the visual contract into
app/javascript/utils/chart_tooltip.js as CHART_TOOLTIP_CLASSES and have both
controllers reference it. Each keeps its own behavioural classes (time-series
its initial `opacity-0`; both `top-0`; sankey toggles opacity via inline
style). `privacy-sensitive` stays bundled so future copies can't drop it.
Also exports a createChartTooltip factory for the raw-DOM idiom.
goal_projection_chart_controller is not in main yet (it lands with the goals
work in #1798); it migrates to the same symbol there.
