Add devices screen to the Flutter frontend

Lists devices with New/Registered/All filtering, pull-to-refresh,
device-type icons, and visual differentiation between new and registered
devices. Introduces a reusable StatusBadge widget for colour-coded labels.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/lib/model/device.dart

@@ -0,0 +1,28 @@
+class Device {
+  final String macAddress;
+  final String ipv4Address;
+  final String vendor;
+  final DateTime lastSeen;
+  final bool isRegistered;
+  final String owner;
+  final String deviceType;
+
+  Device({
+    required this.macAddress,
+    required this.ipv4Address,
+    required this.vendor,
+    required this.lastSeen,
+    required this.isRegistered,
+    required this.owner,
+    required this.deviceType,
+  });
+
+  Device.fromJson(Map<String, dynamic> json)
+    : macAddress = json['mac_address'] as String,
+      ipv4Address = json['ipv4_address'] as String,
+      vendor = json['vendor'] as String,
+      lastSeen = DateTime.parse(json['last_seen'] as String),
+      isRegistered = json['is_registered'] as bool,
+      owner = json['owner'] as String,
+      deviceType = json['device_type'] as String;
+}