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/app/(admin)/admin/reports/page.tsx

@@ -532,9 +532,9 @@ function StageAnalytics() {
               <Skeleton className="h-[350px]" />
             ) : scoreDistribution ? (
               <ScoreDistributionChart
-                data={scoreDistribution.distribution}
-                averageScore={scoreDistribution.averageScore}
-                totalScores={scoreDistribution.totalScores}
+                data={scoreDistribution.distribution ?? []}
+                averageScore={scoreDistribution.averageScore ?? 0}
+                totalScores={scoreDistribution.totalScores ?? 0}
               />
             ) : null}