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/api_error_test.dart

@@ -6,12 +6,12 @@ void main() {
   final options = RequestOptions(path: '/x');
 
   DioException dio(DioExceptionType type, {int? status}) => DioException(
-        requestOptions: options,
-        type: type,
-        response: status == null
-            ? null
-            : Response(requestOptions: options, statusCode: status),
-      );
+    requestOptions: options,
+    type: type,
+    response: status == null
+        ? null
+        : Response(requestOptions: options, statusCode: status),
+  );
 
   test('maps shape errors', () {
     expect(