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/api/backend_api_test_endpoint_test.dart

@@ -6,13 +6,15 @@ void main() {
   // only its error-mapping path is testable here: an unresolvable host must
   // come back as a non-null, user-facing message rather than throwing. The
   // success path is a documented coverage gap.
-  test('BackendAPI.test returns a user message for an unreachable backend',
-      () async {
-    // Loopback port 1 is closed: connection is refused immediately, keeping the
-    // test hermetic (no DNS, no TLS, no traffic leaving the machine).
-    final result = await BackendAPI.test('http://127.0.0.1:1/api', '');
+  test(
+    'BackendAPI.test returns a user message for an unreachable backend',
+    () async {
+      // Loopback port 1 is closed: connection is refused immediately, keeping the
+      // test hermetic (no DNS, no TLS, no traffic leaving the machine).
+      final result = await BackendAPI.test('http://127.0.0.1:1/api', '');
 
-    expect(result, isNotNull);
-    expect(result, isNotEmpty);
-  });
+      expect(result, isNotNull);
+      expect(result, isNotEmpty);
+    },
+  );
 }