refactor(charts): extract shared chart-tooltip className to one source (#2106)

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.

app/javascript/controllers/time_series_chart_controller.js

@@ -1,5 +1,6 @@
 import { Controller } from "@hotwired/stimulus";
 import * as d3 from "d3";
+import { CHART_TOOLTIP_CLASSES } from "utils/chart_tooltip";
 
 const parseLocalDate = d3.timeParse("%Y-%m-%d");
 
@@ -287,10 +288,8 @@ export default class extends Controller {
     this._d3Tooltip = d3
       .select(`#${this.element.id}`)
       .append("div")
-      .attr(
-        "class",
-        "bg-container text-sm font-sans absolute p-2 border border-secondary rounded-lg pointer-events-none opacity-0 top-0 privacy-sensitive",
-      );
+      // Shared visual contract + this chart's initial-hidden / positioning classes.
+      .attr("class", `${CHART_TOOLTIP_CLASSES} opacity-0 top-0`);
   }
 
   _trackMouseForShowingTooltip() {