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/routes.dart

@@ -0,0 +1,15 @@
+/// Centralised route locations so navigation targets are defined once and
+/// reused by both the router (see `navigation.dart`) and every call site.
+abstract final class Routes {
+  static const String home = '/';
+  static const String devices = '/devices';
+  static const String status = '/status';
+  static const String settings = '/settings';
+  static const String about = '/about';
+
+  /// Path segment for the device-detail route, nested under [devices].
+  static const String deviceDetailSegment = ':macAddress';
+
+  /// Full location for a single device's detail page.
+  static String deviceDetail(String macAddress) => '$devices/$macAddress';
+}