feat(cli): status shows last export age

Reads .relicario/last_backup (written by cmd_backup_export). Format:
'never' for fresh vaults, '4 days ago' otherwise. Closes the
'is my backup stale?' question without leaving the terminal.

crates/relicario-cli/src/helpers.rs

@@ -63,6 +63,26 @@ pub fn iso8601(unix_seconds: i64) -> String {
         .unwrap_or_else(|| format!("invalid-timestamp:{unix_seconds}"))
 }
 
+/// Format a duration (in seconds) as a coarse human-readable string:
+/// "just now" / "5 minutes ago" / "4 days ago" / "3 months ago".
+pub fn humanize_age(seconds: i64) -> String {
+    if seconds < 60 { return "just now".to_string(); }
+    if seconds < 3600 { return format!("{} minute{} ago", seconds / 60, plural(seconds / 60)); }
+    if seconds < 86_400 { return format!("{} hour{} ago", seconds / 3600, plural(seconds / 3600)); }
+    if seconds < 86_400 * 30 {
+        let d = seconds / 86_400;
+        return format!("{d} day{} ago", plural(d));
+    }
+    if seconds < 86_400 * 365 {
+        let m = seconds / (86_400 * 30);
+        return format!("{m} month{} ago", plural(m));
+    }
+    let y = seconds / (86_400 * 365);
+    format!("{y} year{} ago", plural(y))
+}
+
+fn plural(n: i64) -> &'static str { if n == 1 { "" } else { "s" } }
+
 #[cfg(test)]
 mod tests {
     use super::*;