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/helpers/fixtures.dart

@@ -42,7 +42,7 @@ Map<String, dynamic> deviceEventJson({
   int id = 1,
   String macAddress = 'aa:bb:cc:dd:ee:ff',
   String createdOn = '2026-06-01T12:00:00Z',
-  String eventType = 'seen',
+  String eventType = 'DeviceSeen',
   String ipv4Address = '192.168.0.10',
   String vendor = 'Acme Corp',
   String scanner = 'Arp',