Add Status screen with ARP scanner live status display

Shows ARP scanner state (running/waiting/error) with a colored indicator
and time that ticks locally every second, refreshing from the API every 5s.

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

frontend/lib/model/arp_scanner_status.dart

@@ -0,0 +1,19 @@
+class ArpScannerStatus {
+  final bool isRunning;
+  final double? runningForSeconds;
+  final double? nextRunInSeconds;
+
+  const ArpScannerStatus({
+    required this.isRunning,
+    this.runningForSeconds,
+    this.nextRunInSeconds,
+  });
+
+  factory ArpScannerStatus.fromJson(Map<String, dynamic> json) {
+    return ArpScannerStatus(
+      isRunning: json['is_running'] as bool,
+      runningForSeconds: (json['running_for_seconds'] as num?)?.toDouble(),
+      nextRunInSeconds: (json['next_run_in_seconds'] as num?)?.toDouble(),
+    );
+  }
+}