Group metrics dropdown by category in rankings and comparison views

Added optgroup elements to organize metrics into logical categories:
- Expected Standard
- Higher Standard
- Progress Scores
- Average Scores
- Gender Performance
- Equity (Disadvantaged)
- School Context
- 3-Year Trends

Also added CSS styling for optgroup labels to match the design system.

Note: School names in rankings are already clickable links to school details.

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

nextjs-app/components/ComparisonView.module.css

@@ -92,6 +92,19 @@
   box-shadow: 0 0 0 3px rgba(224, 114, 86, 0.15);
 }
 
+.metricSelect optgroup {
+  font-weight: 700;
+  color: var(--text-primary, #1a1612);
+  background: var(--bg-secondary, #f3ede4);
+  padding: 0.5rem 0;
+}
+
+.metricSelect option {
+  font-weight: 400;
+  color: var(--text-secondary, #5c564d);
+  padding: 0.375rem 1rem;
+}
+
 /* Schools Section */
 .schoolsSection {
   margin-bottom: 2rem;