Fix type error for null leftAction in group renderer

Cast null to string since use-kbd's TwoColumnRow type doesn't
yet support null values (runtime handles it fine).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: ryan-williams

www/src/components/groupRenderers.tsx

@@ -30,7 +30,7 @@ export const YAxisMetricsRenderer = createTwoColumnRenderer({
   headers: ['Metric', 'Left', 'Right'],
   getRows: () => METRICS.map(m => ({
     label: METRIC_LABELS[m] ?? m,
-    leftAction: m === 'none' ? null : `left:${m}`,
+    leftAction: (m === 'none' ? null : `left:${m}`) as string,
     rightAction: `right:${m}`,
   })),
 })