Function format_timestamp 

pub fn format_timestamp(timestamp_ms: i64, format: &str) -> String

Format a Unix timestamp (in milliseconds) using the given format string.

Uses chrono format specifiers:

• %Y = year (4 digits)
• %m = month (01-12)
• %d = day (01-31)
• %H = hour (00-23)
• %M = minute (00-59)
• %S = second (00-59)
• %F = ISO date (YYYY-MM-DD)
• %T = ISO time (HH:MM:SS)