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/widget/scanner_status_card_test.dart

@@ -14,8 +14,9 @@ void main() {
     await setUpBackendForTest();
   });
 
-  testWidgets('shows a spinner, then the resolved label and sublabels',
-      (tester) async {
+  testWidgets('shows a spinner, then the resolved label and sublabels', (
+    tester,
+  ) async {
     await pumpScreen(
       tester,
       ScannerStatusCard<int>(