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/test/unit/device_event_test.dart

@@ -1,5 +1,6 @@
 import 'package:flutter_test/flutter_test.dart';
 import 'package:frontend/model/device_event.dart';
+import 'package:frontend/model/device_event_type.dart';
 
 import '../helpers/fixtures.dart';
 
@@ -10,7 +11,7 @@ void main() {
         id: 42,
         macAddress: '11:22:33:44:55:66',
         createdOn: '2026-05-20T10:30:00Z',
-        eventType: 'changed',
+        eventType: 'NewDevice',
         ipv4Address: '10.0.0.9',
         vendor: 'Initech',
         scanner: 'Ssdp',
@@ -20,12 +21,21 @@ void main() {
     expect(event.id, 42);
     expect(event.macAddress, '11:22:33:44:55:66');
     expect(event.createdOn, DateTime.parse('2026-05-20T10:30:00Z'));
-    expect(event.eventType, 'changed');
+    expect(event.eventType, DeviceEventType.newDevice);
     expect(event.ipv4Address, '10.0.0.9');
     expect(event.vendor, 'Initech');
     expect(event.scanner, 'Ssdp');
   });
 
+  test('DeviceEvent.fromJson parses event types and falls back to seen', () {
+    DeviceEventType parsed(String value) =>
+        DeviceEvent.fromJson(deviceEventJson(eventType: value)).eventType;
+
+    expect(parsed('NewDevice'), DeviceEventType.newDevice);
+    expect(parsed('DeviceSeen'), DeviceEventType.deviceSeen);
+    expect(parsed('something_unexpected'), DeviceEventType.deviceSeen);
+  });
+
   test('scannerLabel humanises known scanner names', () {
     expect(_label('Arp'), 'ARP');
     expect(_label('Mdns'), 'mDNS');