fix(compare): give the trend chart its own definite canvas height so the gap note can't squash it

The chart note (and the 2-row mobile chip legend) lived inside the fixed
360px/420px chartBox and competed with the canvas for it, so a longer
caption — e.g. the new KS4 gap note — shrank the mobile canvas to ~178px
and tripped the e2e >220px guard. The canvas now owns a definite height
(280px mobile / 380px desktop) with chips above and the note flowing
below at natural size, so neither can eat the plot. Verified on staging:
patched canvas renders 280px vs the squashed 178px.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/components/ComparisonChart.module.css

@@ -1,16 +1,23 @@
-/* Chart wrapper: chips (mobile) above, canvas filling the rest of the
-   parent .chartContainer, whose fixed height drives Chart.js sizing via
-   maintainAspectRatio: false. */
+/* Chart wrapper: chips (mobile) above, then the canvas, then the gap note.
+   The canvas has its OWN definite height (Chart.js needs one for
+   maintainAspectRatio: false); the chips and the note flow at their natural
+   size around it rather than competing with it for a fixed outer height —
+   so a longer note (e.g. the KS4 gap caption) or a two-row chip legend can
+   never squash the chart. */
 .wrapper {
     display: flex;
     flex-direction: column;
-    height: 100%;
 }
 
 .canvasBox {
     position: relative;
-    flex: 1 1 auto;
-    min-height: 0;
+    height: 380px;
+}
+
+@media (max-width: 640px) {
+    .canvasBox {
+        height: 280px;
+    }
 }
 
 /* School chips: mobile-only legend + tap-to-focus control. Desktop keeps