Add reopen button for closed rounds

Allows admins to reopen a closed round. When reopened, the voting start
date will automatically update to now (if it was in the future).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/(admin)/admin/rounds/[id]/page.tsx

@@ -300,6 +300,15 @@ function RoundDetailContent({ roundId }: { roundId: string }) {
               Close Round
             </Button>
           )}
+          {round.status === 'CLOSED' && (
+            <Button
+              onClick={() => updateStatus.mutate({ id: round.id, status: 'ACTIVE' })}
+              disabled={updateStatus.isPending}
+            >
+              <Play className="mr-2 h-4 w-4" />
+              Reopen Round
+            </Button>
+          )}
           <AlertDialog>
             <AlertDialogTrigger asChild>
               <Button variant="destructive">