formatAgo
Formats a given Unix timestamp (in seconds) into a relative "time ago" string.
For durations less than a minute, it returns "now". For longer durations, it uses DateFormatter to generate a concise, localized representation (e.g., "5m ago", "2h ago").
Return
A String representing the relative time that has passed.
Parameters
lastSeenUnixSeconds
The Unix timestamp in seconds to be formatted.