Fix observer analytics crash: guard Nivo edge cases

- Disable enableSlices on ResponsiveLine with single data point (causes
  null reference in Nivo internal slice computation)
- Add null check for slice.points[0] in timeline tooltip
- Guard ResponsivePie from empty data array in diversity metrics
- Add fallback for scoreDistribution.distribution on both
  observer and admin reports pages

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/charts/evaluation-timeline.tsx

@@ -63,9 +63,10 @@ export function EvaluationTimelineChart({ data }: EvaluationTimelineProps) {
             pointBorderWidth={2}
             pointBorderColor="#ffffff"
             useMesh={true}
-            enableSlices="x"
+            enableSlices={formattedData.length >= 2 ? 'x' : false}
             sliceTooltip={({ slice }) => {
               const point = slice.points[0]
+              if (!point) return null
               const dataItem = formattedData.find(
                 (d) => d.dateFormatted === point.data.xFormatted
               )