feat(charts): give sankey and goal tooltips their missing context rows

Chart tooltips answer three stacked questions: context (what am I
looking at), value (how much), relation (vs what). Time-series already
had all three; the other two were missing rows.

- Sankey links showed a bare "$X (Y%)" with no indication of which
  flow was hovered. Links now lead with a swatch-dotted
  "Source → Target" context line; nodes get the same dot + name
  treatment, tying the card to the ribbon color. The context builder
  escapes node names centrally (they're user-named categories).
- Goal projection adds a tertiary relation line — "52% of $20K
  target" — computed from the payload the chart already carries, for
  both the saved and projected segments. Hidden when the goal has no
  positive target. Template is i18n-wired like the existing tooltip
  strings (goals.show.projection.tooltip_target_relation).

Verified with Playwright against the running app: all three surfaces
pass computed-style and content assertions.

app/javascript/controllers/goal_projection_chart_controller.js

@@ -23,6 +23,7 @@ export default class extends Controller {
     todayLabel: { type: String, default: "Today" },
     projectedTemplate: { type: String, default: "Projected: {amount}" },
     savedTemplate: { type: String, default: "Saved: {amount}" },
+    targetRelationTemplate: { type: String, default: "{percent}% of {target} target" },
   };
 
   connect() {
@@ -452,7 +453,25 @@ export default class extends Controller {
     tooltipDate.className = CHART_TOOLTIP_CONTEXT_CLASSES;
     const tooltipValue = document.createElement("div");
     tooltipValue.className = CHART_TOOLTIP_VALUE_CLASSES;
-    tooltip.replaceChildren(tooltipDate, tooltipValue);
+    // Relation line: where this value sits against the goal target. Tertiary
+    // so the hierarchy stays date < value > relation; hidden when the goal
+    // has no positive target to compare against.
+    const tooltipRelation = document.createElement("div");
+    tooltipRelation.className = "text-xs text-subdued mt-0.5";
+    tooltip.replaceChildren(tooltipDate, tooltipValue, tooltipRelation);
+
+    const setRelation = (amount) => {
+      const target = Number(data.target_amount) || 0;
+      if (target <= 0 || !data.target_amount_short_label) {
+        tooltipRelation.style.display = "none";
+        return;
+      }
+      const percent = Math.round((amount / target) * 100);
+      tooltipRelation.textContent = this.targetRelationTemplateValue
+        .replace("{percent}", percent)
+        .replace("{target}", data.target_amount_short_label);
+      tooltipRelation.style.display = "";
+    };
 
     const overlay = svg
       .append("rect")
@@ -505,6 +524,7 @@ export default class extends Controller {
         hoverProjDot.attr("cx", hoverX).attr("cy", y(projValue)).style("display", null);
         hoverSavedDot.style("display", "none");
         tooltipValue.textContent = this.projectedTemplateValue.replace("{amount}", this._fmtMoney(projValue, data.currency));
+        setRelation(projValue);
       } else {
         // Saved segment: hoverDate is already snapped to nearest savedSeries
         // entry above, so reuse that entry directly instead of running
@@ -513,6 +533,7 @@ export default class extends Controller {
         hoverSavedDot.attr("cx", x(savedPoint.date)).attr("cy", y(savedPoint.value)).style("display", null);
         hoverProjDot.style("display", "none");
         tooltipValue.textContent = this.savedTemplateValue.replace("{amount}", this._fmtMoney(savedPoint.value, data.currency));
+        setRelation(savedPoint.value);
       }
 
       tooltip.style.display = "block";

app/javascript/controllers/sankey_chart_controller.js

@@ -445,7 +445,17 @@ export default class extends Controller {
     linkPaths
       .on("mouseenter", (event, d) => {
         applyHover([d]);
-        this.#showTooltip(event, d.value, d.percentage);
+        // A link is a flow between two named nodes — without the names the
+        // value floats context-free (the old tooltip showed only "$X (Y%)").
+        this.#showTooltip(
+          event,
+          d.value,
+          d.percentage,
+          this.#tooltipContext(
+            d.source.color,
+            `${this.#esc(d.source.name)} → ${this.#esc(d.target.name)}`,
+          ),
+        );
       })
       .on("mousemove", (event) => this.#updateTooltipPosition(event))
       .on("mouseleave", () => {
@@ -466,7 +476,12 @@ export default class extends Controller {
           (l) => l.source === d || l.target === d,
         );
         applyHover(connectedLinks);
-        this.#showTooltip(event, d.value, d.percentage, d.name);
+        this.#showTooltip(
+          event,
+          d.value,
+          d.percentage,
+          this.#tooltipContext(d.color, this.#esc(d.name)),
+        );
       })
       .on("mousemove", (event) => this.#updateTooltipPosition(event))
       .on("click", (event, d) => {
@@ -490,7 +505,12 @@ export default class extends Controller {
           (l) => l.source === d || l.target === d,
         );
         applyHover(connectedLinks);
-        this.#showTooltip(event, d.value, d.percentage, d.name);
+        this.#showTooltip(
+          event,
+          d.value,
+          d.percentage,
+          this.#tooltipContext(d.color, this.#esc(d.name)),
+        );
       })
       .on("mousemove", (event) => this.#updateTooltipPosition(event))
       .on("click", (event, d) => {
@@ -517,20 +537,29 @@ export default class extends Controller {
       .style("pointer-events", "none");
   }
 
-  #showTooltip(event, value, percentage, title = null) {
+  // Node names are user-named categories; escape anything interpolated into
+  // .html() (the previous code injected them raw).
+  #esc(s) {
+    return String(s).replace(
+      /[&<>"']/g,
+      (c) =>
+        ({ "&": "&amp;", "<": "&lt;", ">": "&gt;", '"': "&quot;", "'": "&#39;" })[c],
+    );
+  }
+
+  // Context line shared by node and link tooltips: a swatch dot tying the
+  // card to the hovered ribbon/node color, plus the (escaped) name(s).
+  #tooltipContext(color, label) {
+    const dot = `<span class="inline-block shrink-0 rounded-full" style="width: 8px; height: 8px; background: ${this.#esc(color || "var(--color-gray-400)")};"></span>`;
+    return `<div class="flex items-center gap-1.5 text-xs text-secondary mb-1">${dot}<span class="min-w-0 truncate">${label}</span></div>`;
+  }
+
+  #showTooltip(event, value, percentage, contextHtml = null) {
     if (!this.tooltip) this.#createTooltip();
 
-    // Node titles are user-named categories; escape them since this goes
-    // through .html() (the previous interpolation injected them raw).
-    const esc = (s) =>
-      String(s).replace(
-        /[&<>"']/g,
-        (c) =>
-          ({ "&": "&amp;", "<": "&lt;", ">": "&gt;", '"': "&quot;", "'": "&#39;" })[c],
-      );
     const valueLine = `<span class="font-mono font-medium tabular-nums">${this.#formatCurrency(value)}</span> <span class="text-secondary">(${percentage || 0}%)</span>`;
-    const content = title
-      ? `<div class="text-xs text-secondary mb-1">${esc(title)}</div><div>${valueLine}</div>`
+    const content = contextHtml
+      ? `${contextHtml}<div>${valueLine}</div>`
       : valueLine;
 
     const isInDialog = !!this.element.closest("dialog");