fix(savings): refine hero spacing, goal/account card padding, sparkline negative range

- Sparkline (`savings-sparkline` controller): dropped the `Math.max(0,
  yMin)` clamp on the y-axis domain so negative balances (or any series
  that dips into negative territory) render fully instead of being
  cropped off the canvas.
- Hero card: padding `p-6` → `p-7`, column ratio
  `[minmax(0,1fr)_minmax(0,1.6fr)]` so the chart breathes, min height
  bumped to 220px, sparkline container `h-full min-h-[200px]` so it
  fills the card vertically. Stats row now sits at the bottom of the
  text column via `mt-auto pt-6`; labels promoted to `text-xs`, values
  to `text-lg`.
- Section vertical rhythm: outer `space-y-6` → `space-y-8`.
- Goal card: padding `p-[18px]` → `p-6`. Internal gap from header row
  to amount line `mt-3.5` → `mt-5`. Account-row gap `mt-3` → `mt-4`.
- Account card: padding `p-5` → `p-6`.
- Status pill "Behind" dot: `bg-yellow-500` → `bg-yellow-600` for a
  warmer/ambery tone matching the Claude Design reference.
- Goal card donut "behind" stroke: `var(--color-yellow-500)` →
  `var(--color-yellow-600)` to match the pill.

app/javascript/controllers/savings_sparkline_controller.js

@@ -42,10 +42,13 @@ export default class extends Controller {
 
     const yMin = Math.min(...series.map((d) => d.value));
     const yMax = Math.max(...series.map((d) => d.value));
-    const padding = (yMax - yMin) * 0.15 || yMax * 0.05 || 1;
+    // Don't clamp to 0 — savings totals can be negative under certain
+    // demo / edge conditions, and clamping pushes the line off-canvas.
+    const range = yMax - yMin;
+    const padding = range > 0 ? range * 0.15 : Math.abs(yMax) * 0.05 || 1;
     const y = d3
       .scaleLinear()
-      .domain([Math.max(0, yMin - padding), yMax + padding])
+      .domain([yMin - padding, yMax + padding])
       .range([margin.top + innerHeight, margin.top]);
 
     const svg = d3