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/diversity-metrics.tsx

@@ -115,7 +115,7 @@ export function DiversityMetricsChart({ data }: DiversityMetricsProps) {
           </CardHeader>
           <CardContent>
             <div style={{ height: '400px' }}>
-              <ResponsivePie
+              {nivoPieData.length > 0 ? <ResponsivePie
                 data={nivoPieData}
                 theme={nivoTheme}
                 colors={[...BRAND_COLORS]}
@@ -149,7 +149,9 @@ export function DiversityMetricsChart({ data }: DiversityMetricsProps) {
                     symbolShape: 'circle',
                   },
                 ]}
-              />
+              /> : (
+                <p className="text-muted-foreground text-center py-8">No geographic data</p>
+              )}
             </div>
           </CardContent>
         </Card>