Function is_idle_at 

pub fn is_idle_at(last_activity: Option<SystemTime>, idle_days: u64) -> bool

Whether an already-known activity time counts as idle.

Split out from is_repo_idle so a caller that has just computed the activity time for display can decide idleness from the same value instead of recomputing it. The dashboard used to do exactly that — a second git log plus a second full tree walk per repository — and, worse, showed a “last activity” that the idle decision had not actually used.