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/home/notification_card.dart

@@ -5,6 +5,7 @@ import '../model/notification.dart' as oott_model;
 import '../theme/app_colors.dart';
 import '../theme/dimens.dart';
 import '../utils/friendly_date_formatter.dart';
+import '../routes.dart';
 
 // How long the arrival highlight takes to fade out.
 const _flashDuration = Duration(milliseconds: 900);
@@ -67,7 +68,7 @@ class _NotificationCardState extends State<NotificationCard> {
                   if (left) widget.onRemove?.call(animated: true);
                 }
                 if (context.mounted) {
-                  context.push('/devices/${widget.item.macAddress}');
+                  context.push(Routes.deviceDetail(widget.item.macAddress!));
                 }
               },
             ),