feat(02-01): register Ranking tab in round detail page + create component stub

- Create src/components/admin/round/ranking-dashboard.tsx (stub with placeholder text)
- Import RankingDashboard in round detail page.tsx
- Add Ranking tab entry guarded by isEvaluation (uses existing BarChart3 icon)
- Add TabsContent block rendering RankingDashboard with competitionId + roundId props

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

@@ -0,0 +1,14 @@
+'use client'
+
+type RankingDashboardProps = {
+  competitionId: string
+  roundId: string
+}
+
+export function RankingDashboard({ competitionId: _competitionId, roundId: _roundId }: RankingDashboardProps) {
+  return (
+    <div className="text-muted-foreground text-sm p-4">
+      Ranking dashboard coming soon...
+    </div>
+  )
+}