A 38-year-old man with chronic alcohol use disorder presents with jaundice, hepatomegaly, and elevated transaminases (AST 180 U/L, ALT 95 U/L). Liver biopsy shows hepatocyte ballooning, Mallory-Denk bodies, and neutrophilic infiltration consistent with alcoholic hepatitis. Which drug is the first-line treatment to prevent progression to irreversible cirrhosis?