Add loading state and debugging for comparison chart

Added better UX and debugging for the comparison screen:

1. Loading state for chart section
   - Shows "Loading comparison data..." when schools are selected
     but data hasn't loaded yet
   - Provides visual feedback to users

2. Enhanced debugging logs
   - Log URNs being fetched
   - Log API response status
   - Log received comparison data
   - Better error handling with null state on failure

3. Improved conditional rendering
   - Chart shows when data is available
   - Loading message shows when waiting for data
   - Nothing shows when no schools selected

This helps diagnose any API issues and provides better user feedback
during data loading.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

nextjs-app/components/ComparisonView.module.css

@@ -215,6 +215,13 @@
   position: relative;
 }
 
+.loadingMessage {
+  text-align: center;
+  padding: 3rem;
+  color: var(--text-secondary);
+  font-size: 1rem;
+}
+
 /* Table Section */
 .tableSection {
   background: white;