Display unknown values as a dash and centralise repeated literals

Backend: notifications now show a plain "-" for an absent name, vendor, or
device type (was empty string / "(unknown)" / "Unknown"), via a single
UNKNOWN_PLACEHOLDER constant.

Frontend:
- Empty/unknown values render as an em dash everywhere, centralised in a new
  Placeholders.emptyValue constant (replaces inline '—' and '(unknown)').
- Route paths moved to a new Routes class, used by the router and every
  navigation call site.
- Device event type modelled as a DeviceEventType enum mirroring the backend
  (NewDevice/DeviceSeen) instead of bare string comparisons.
- Hardcoded EdgeInsets/SizedBox spacing replaced with existing Insets tokens.

Tests and formatting updated; all backend and frontend tests pass.

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

frontend/lib/devices/device_list.dart

@@ -17,6 +17,7 @@ import '../widgets/skeleton.dart';
 import 'device_list_filter.dart';
 import 'device_list_rows.dart';
 import 'device_list_sort.dart';
+import '../routes.dart';
 
 class DeviceList extends StatefulWidget {
   const DeviceList({super.key});
@@ -273,7 +274,7 @@ class _DeviceListState extends State<DeviceList>
         icon: Icons.devices_other_outlined,
         message: _emptyMessage(),
         actionLabel: 'Check scanner status',
-        onAction: () => context.go('/status'),
+        onAction: () => context.go(Routes.status),
       );
     }