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/main.dart

@@ -25,7 +25,9 @@ void main() {
       try {
         await PrefUtil.init();
       } catch (e, stack) {
-        debugPrint('PrefUtil.init failed, continuing with defaults: $e\n$stack');
+        debugPrint(
+          'PrefUtil.init failed, continuing with defaults: $e\n$stack',
+        );
       }
 
       runApp(const MainApp());