fix(compare): give the trends chart a real height (was squashed to ~150px)

ComparisonChart runs Chart.js with maintainAspectRatio:false, so it sizes
to its container's height — which must be definite. TrendsExplorer gave
.chartBox a min-height, which doesn't resolve the chart wrapper's
height:100%, so Chart.js fell back to its ~150px default: a squashed
8.6:1 sliver that didn't match the mockups. Set a definite height (420px
desktop, 360px mobile where the chips row sits above the canvas).

Verified on staging by patching the live height: canvas went from
1287x150 to 1287x392 (desktop) / 284 (mobile) — proper ~3:1 proportions
matching the mockup, with the England dashed line, COVID/2021-22 gap and
table all reading correctly.

An e2e guard asserts the trends canvas is taller than 220px so the
squash can't regress.

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

e2e/tests/journeys.spec.ts

@@ -213,7 +213,12 @@ test('compare chart on mobile shows school chips with tap-to-focus', async ({ pa
   expect(bodyOverflowsX).toBe(false);
 
   // The trends chart still renders (inside the Explore trends section)…
-  await expect(page.locator('canvas:visible').first()).toBeVisible({ timeout: 15_000 });
+  const chartCanvas = page.locator('canvas:visible').first();
+  await expect(chartCanvas).toBeVisible({ timeout: 15_000 });
+  // …at a real height, not the squashed ~150px Chart.js fallback that
+  // appears when the container lacks a definite height.
+  const chartBox = await chartCanvas.boundingBox();
+  expect(chartBox && chartBox.height).toBeGreaterThan(220);
 
   // …with the mobile chart legend chips and tap-to-focus behaviour intact.
   const chipGroup = page.getByRole('group', { name: /highlight a school/i });