Add device event history chart to device details page

Displays a scatter chart timeline of when the device was seen online,
with a time range selector (Today → Last Year). Tooltips show event
date/time, type, and highlight any IP or vendor changes relative to
the current device data.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/lib/devices/device_detail.dart

@@ -6,6 +6,7 @@ import '../utils/friendly_date_formatter.dart';
 import '../utils/oott_api.dart';
 import '../widgets/status_badge.dart';
 import 'device_actions.dart';
+import 'device_event_history.dart';
 
 class DeviceDetail extends StatefulWidget {
   final String macAddress;
@@ -105,6 +106,10 @@ class _DeviceDetailState extends State<DeviceDetail> {
                   _DeviceHeader(device: device),
                   const SizedBox(height: 24),
                   _DeviceInfoCard(device: device),
+                  const SizedBox(height: 24),
+                  _SectionHeader(title: 'Event History'),
+                  const SizedBox(height: 12),
+                  DeviceEventHistory(device: device),
                 ],
               ),
             ),
@@ -112,6 +117,20 @@ class _DeviceDetailState extends State<DeviceDetail> {
   }
 }
 
+class _SectionHeader extends StatelessWidget {
+  final String title;
+
+  const _SectionHeader({required this.title});
+
+  @override
+  Widget build(BuildContext context) {
+    return Text(
+      title,
+      style: Theme.of(context).textTheme.titleMedium,
+    );
+  }
+}
+
 class _DeviceHeader extends StatelessWidget {
   final Device device;