Add device detail screen and navigate to it from notifications and devices

- New /devices/:macAddress route showing all device fields with Register/Forget actions
- Device list items are now tappable; popup menu gains a "View details" item
- Notification list items with a linked device are tappable; popup menu gains a "View device" item
- Backend: new DB migration adds optional mac_address column to notifications
- Device-related notifications (NewDeviceFound, DeviceOnlineAfterTime, DeviceChanged) now store the triggering device's MAC address

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

frontend/lib/navigation.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:frontend/settings/settings.dart';
 import 'package:go_router/go_router.dart';
+import 'devices/device_detail.dart';
 import 'devices/device_list.dart';
 import 'notifications/notification_list.dart';
 import 'utils/pref_utils.dart';
@@ -26,6 +27,16 @@ final GoRouter router = GoRouter(
           name: 'devices',
           builder: (context, state) => const DeviceList(),
           redirect: (context, state) => _redirectToSettings(),
+          routes: [
+            GoRoute(
+              path: ':macAddress',
+              name: 'deviceDetail',
+              builder: (context, state) {
+                final mac = state.pathParameters['macAddress']!;
+                return DeviceDetail(macAddress: mac);
+              },
+            ),
+          ],
         ),
         GoRoute(
           path: '/settings',