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/test/unit/duration_formatter_test.dart

@@ -21,6 +21,26 @@ void main() {
     expect(formatSeconds(59.6), '1m 0s');
   });
 
+  test('renders hour-and-minute values past 60 minutes', () {
+    expect(formatSeconds(3600), '1h 0m');
+    expect(formatSeconds(3600 + 125), '1h 2m');
+  });
+
+  test('renders day-and-hour values past 24 hours', () {
+    expect(formatSeconds(86400), '1d 0h');
+    expect(formatSeconds(86400 + 3 * 3600), '1d 3h');
+  });
+
+  test('renders week-and-day values past 7 days', () {
+    expect(formatSeconds(604800), '1w 0d');
+    expect(formatSeconds(604800 + 2 * 86400), '1w 2d');
+  });
+
+  test('renders month-and-week values past 30 days', () {
+    expect(formatSeconds(2592000), '1mo 0w');
+    expect(formatSeconds(2592000 + 14 * 86400), '1mo 2w');
+  });
+
   test('clamps negative values to zero', () {
     expect(formatSeconds(-10), '0s');
   });