Scale duration formatting up to months

formatSeconds previously capped at minutes; extend it to dynamically
show the two largest relevant units up through months for the passive
scanners' "Listening for ..." line and other duration displays.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/utils/duration_formatter.dart

@@ -1,7 +1,20 @@
+/// Formats a duration given in [seconds] into a compact, human-readable string.
+///
+/// The format scales with the magnitude of the duration, always showing the two
+/// largest relevant units: seconds for sub-minute spans, then minutes, hours,
+/// days, weeks and finally months for longer ones.
 String formatSeconds(double seconds) {
   final total = seconds.round().clamp(0, double.maxFinite.toInt());
-  if (total < 60) return '${total}s';
-  final m = total ~/ 60;
-  final s = total % 60;
-  return '${m}m ${s}s';
+  const minute = 60;
+  const hour = 60 * minute;
+  const day = 24 * hour;
+  const week = 7 * day;
+  const month = 30 * day;
+
+  if (total < minute) return '${total}s';
+  if (total < hour) return '${total ~/ minute}m ${total % minute}s';
+  if (total < day) return '${total ~/ hour}h ${(total % hour) ~/ minute}m';
+  if (total < week) return '${total ~/ day}d ${(total % day) ~/ hour}h';
+  if (total < month) return '${total ~/ week}w ${(total % week) ~/ day}d';
+  return '${total ~/ month}mo ${(total % month) ~/ week}w';
 }