fix(savings_goals): ring on_track color, contributions horizontal scroll, modal restored on back, chart saved fill

Ring on on_track / no_target_date goal cards rendered with no progress
arc — ring_color returned var(--text-primary) / var(--text-subdued)
which aren't real CSS custom properties (Sure's text tokens are Tailwind
utilities). Switch to var(--color-green-500) / var(--color-gray-400)
which ARE CSS vars from the Tailwind palette and resolve at SVG fill
time.

Contributions list had a horizontal scrollbar because the rows used
-mx-3 px-3 to extend the hover background, which pushed content beyond
the card padding. Drop the negative-margin trick and add
overflow-x-hidden to the scroll container. Rows still hover-highlight
inside the card bounds.

Modal cache restoration — Turbo cached pages with open <dialog> elements
inside <turbo-frame id="modal">. After dismissing the new-goal modal
and navigating to a goal detail page, browser back restored the cached
index page WITH the dialog still in the modal frame; the dialog's
Stimulus controller then ran auto-open and reopened it. Now the dialog
close handler empties the parent modal turbo-frame so the cache
snapshot is clean.

Chart saved-fill — bump area gradient stop-opacity 0.10 → 0.22 so the
contribution history is more visible against the dark canvas. Chart
was rendering correctly but the white-at-10%-opacity gradient was too
faint to read on top of the dashed projection.

app/javascript/controllers/savings_goal_projection_chart_controller.js

@@ -86,7 +86,7 @@ export default class extends Controller {
       .append("linearGradient")
       .attr("id", `saved-fill-${this._id()}`)
       .attr("x1", 0).attr("y1", 0).attr("x2", 0).attr("y2", 1);
-    gradient.append("stop").attr("offset", "0%").attr("stop-color", textPrimary).attr("stop-opacity", 0.10);
+    gradient.append("stop").attr("offset", "0%").attr("stop-color", textPrimary).attr("stop-opacity", 0.22);
     gradient.append("stop").attr("offset", "100%").attr("stop-color", textPrimary).attr("stop-opacity", 0);
 
     if (targetAmount > 0) {