A 52-year-old man with chronic alcohol use presents with jaundice, hepatomegaly, and elevated transaminases (ALT 180 U/L, AST 240 U/L). Liver biopsy shows hepatocyte ballooning, Mallory–Denk bodies, and neutrophilic infiltration consistent with acute alcoholic hepatitis. Which drug is the first-line treatment to prevent progression to irreversible liver injury (cirrhosis)?