Show scanner source in device event tooltip

Surface the originating scanner ("ARP" or "mDNS") in the event timeline
tooltip so users can tell which scanner reported each sighting.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/lib/devices/device_event_history.dart

@@ -257,9 +257,10 @@ class _EventChart extends StatelessWidget {
               final event = events[idx];
               final dt = event.createdOn.toLocal();
               final dateStr = DateFormat('MMM d, yyyy HH:mm').format(dt);
-              final typeLabel = event.eventType == 'NewDevice'
+              final baseLabel = event.eventType == 'NewDevice'
                   ? 'First seen'
                   : 'Device seen';
+              final typeLabel = '$baseLabel (${event.scannerLabel})';
 
               final diffs = <TextSpan>[];
               if (event.ipv4Address != device.ipv4Address) {