style: increase Trust Comparison chart height from 320px to 500px (Task 11.2)

Charts were overspilling 320px containers. Increased to 500px for
readability. Added overflow: hidden + min-height: 0 to .tc-chart-cell
to prevent any content leaking. Dashboard scrolls vertically, which
is acceptable for 6 readable charts.

dash_app/assets/nhs.css

@@ -467,6 +467,8 @@ body {
     border: 1px solid var(--nhs-pale-grey);
     display: flex;
     flex-direction: column;
+    overflow: hidden;
+    min-height: 0;
 }
 .tc-chart-cell__title {
     padding: 10px 16px;