feat: mount score explainer dialog in admin and observer surfaces

Adds the 'How scores are calculated' affordance to:
- the admin ranking dashboard side panel (next to the Avg Score card)
- the observer full project detail page (in the score card)
- the observer reports preview dialog (next to Evaluation Summary)

so all three audiences can open the same explainer dialog.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/admin/round/ranking-dashboard.tsx

@@ -38,6 +38,7 @@ import { Label } from '@/components/ui/label'
 import { Textarea } from '@/components/ui/textarea'
 import { Slider } from '@/components/ui/slider'
 import { Switch } from '@/components/ui/switch'
+import { ScoreExplainerDialog } from '@/components/shared/score-explainer-dialog'
 import {
   Collapsible,
   CollapsibleContent,
@@ -1075,6 +1076,9 @@ export function RankingDashboard({ competitionId: _competitionId, roundId }: Ran
                           {useBalanced && <span className="ml-1 text-[10px] text-muted-foreground">← used for ranking</span>}
                         </div>
                       </div>
+                      <div className="mt-2 flex justify-end">
+                        <ScoreExplainerDialog />
+                      </div>
                     </div>
                     <div className="grid grid-cols-2 gap-3">
                       <div className="rounded-lg border p-3 text-center">