Fix metrics API response structure

Backend returns metrics as an array, not an object.
- Update MetricsResponse type to use MetricDefinition[] instead of Record
- Remove Object.values() conversion in compare and rankings pages
- Fix useMetrics hook to handle array instead of object
- Fix getMetric to use array.find() instead of object indexing

Fixes empty metric dropdown on compare page.

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

nextjs-app/lib/types.ts

@@ -194,8 +194,9 @@ export interface MetricDefinition {
   hasNationalAverage?: boolean;
 }
 
+// Backend returns metrics as an array, not an object
 export interface MetricsResponse {
-  metrics: Record<string, MetricDefinition>;
+  metrics: MetricDefinition[];
 }
 
 export interface DataInfoResponse {