fix(chart): stop "View raw data" link overlapping the mobile chart canvas

The previous commit added the chip strip and subtitle inside the
.chartOuter, but .chartContainer on the parent SchoolDetailView still
had a fixed height: 220px. With the new content stacking above the
canvas the chart + COVID footnote overflowed past 220px, and the
<details> "View raw year-by-year data" disclosure (rendered just
after the container) landed on top of the plot. It also pushed the
COVID footnote out of the card onto the page background.

- .chartContainer at ≤640px now flows naturally (height: auto)
- .chartWrapper at ≤640px gets an explicit 220px height so the canvas
  itself still has a known size for Chart.js to render into.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nextjs-app/components/SchoolDetailView.module.css

@@ -880,8 +880,12 @@
         grid-template-columns: 1fr;
     }
 
+    /* PerformanceChart on mobile now stacks: trend banner → chip subtitle →
+       chip row → canvas → mini-legend → COVID footnote. The container must
+       flow naturally instead of clipping to a fixed 220px — PerformanceChart's
+       own .chartWrapper carries the canvas height. */
     .chartContainer {
-        height: 220px;
+        height: auto;
     }
 
     .mapContainer {