fix(compare): keep chart data when a client refetch fails

The compare view refetches /api/compare on the client after SSR; on any
failure the catch nulled comparisonData, destroying the working
SSR-provided chart. A transient error (or staging's broken external /api
proxy) should not blank a comparison the user is already viewing — keep
the existing data instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

nextjs-app/components/ComparisonView.tsx

@@ -111,8 +111,10 @@ export function ComparisonView({
           setComparisonData(data.comparison);
         })
         .catch((err) => {
+          // Keep whatever we already have (SSR data or a previous fetch) rather
+          // than blanking the chart — a transient refetch failure shouldn't
+          // destroy a working comparison the user is looking at.
           console.error('Failed to fetch comparison:', err);
-          setComparisonData(null);
         });
     } else {
       setComparisonData(null);