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/status/status_screen.dart

@@ -0,0 +1,165 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:frontend/model/arp_scanner_status.dart';
+import 'package:frontend/utils/oott_api.dart';
+
+class StatusScreen extends StatefulWidget {
+  const StatusScreen({super.key});
+
+  @override
+  State<StatusScreen> createState() => _StatusScreenState();
+}
+
+class _StatusScreenState extends State<StatusScreen> {
+  ArpScannerStatus? _status;
+  DateTime? _statusReceivedAt;
+  bool _isLoading = true;
+  String? _error;
+  Timer? _refreshTimer;
+  Timer? _tickTimer;
+
+  @override
+  void initState() {
+    super.initState();
+    _loadStatus();
+    _refreshTimer = Timer.periodic(
+      const Duration(seconds: 5),
+      (_) => _loadStatus(),
+    );
+    _tickTimer = Timer.periodic(
+      const Duration(seconds: 1),
+      (_) { if (mounted) setState(() {}); },
+    );
+  }
+
+  @override
+  void dispose() {
+    _refreshTimer?.cancel();
+    _tickTimer?.cancel();
+    super.dispose();
+  }
+
+  Future<void> _loadStatus() async {
+    try {
+      final status = await BackendAPI.instance.getArpScannerStatus();
+      if (!mounted) return;
+      setState(() {
+        _status = status;
+        _statusReceivedAt = DateTime.now();
+        _error = null;
+        _isLoading = false;
+      });
+    } catch (e) {
+      if (!mounted) return;
+      setState(() {
+        _error = e.toString();
+        _isLoading = false;
+      });
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: [
+        Text('Status', style: Theme.of(context).textTheme.headlineMedium),
+        const SizedBox(height: 20),
+        if (_isLoading)
+          const Center(child: CircularProgressIndicator())
+        else
+          _ArpScannerCard(
+            status: _status,
+            statusReceivedAt: _statusReceivedAt,
+            error: _error,
+          ),
+      ],
+    );
+  }
+}
+
+class _ArpScannerCard extends StatelessWidget {
+  final ArpScannerStatus? status;
+  final DateTime? statusReceivedAt;
+  final String? error;
+
+  const _ArpScannerCard({this.status, this.statusReceivedAt, this.error});
+
+  @override
+  Widget build(BuildContext context) {
+    final (color, label, sublabel) = _resolveState(context);
+
+    return Card(
+      child: Padding(
+        padding: const EdgeInsets.all(16),
+        child: Row(
+          children: [
+            Icon(Icons.circle, color: color, size: 14),
+            const SizedBox(width: 12),
+            Expanded(
+              child: Column(
+                crossAxisAlignment: CrossAxisAlignment.start,
+                children: [
+                  Text(
+                    'ARP Scanner',
+                    style: Theme.of(context).textTheme.titleMedium,
+                  ),
+                  const SizedBox(height: 2),
+                  Text(label, style: Theme.of(context).textTheme.bodyMedium),
+                  if (sublabel != null) ...[
+                    const SizedBox(height: 2),
+                    Text(
+                      sublabel,
+                      style: Theme.of(context).textTheme.bodySmall?.copyWith(
+                        color: Theme.of(context).colorScheme.outline,
+                      ),
+                    ),
+                  ],
+                ],
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+
+  (Color, String, String?) _resolveState(BuildContext context) {
+    if (error != null || status == null) {
+      return (
+        Colors.red,
+        'Error',
+        'Unable to reach the server or a server-side error occurred. Check the logs for details.',
+      );
+    }
+
+    final elapsed = statusReceivedAt != null
+        ? DateTime.now().difference(statusReceivedAt!).inSeconds.toDouble()
+        : 0.0;
+
+    if (status!.isRunning) {
+      final sub = status!.runningForSeconds != null
+          ? 'Running for ${_formatSeconds(status!.runningForSeconds! + elapsed)}'
+          : null;
+      return (Colors.green, 'Running', sub);
+    }
+    if (status!.nextRunInSeconds != null) {
+      final remaining = (status!.nextRunInSeconds! - elapsed).clamp(0.0, double.infinity);
+      return (
+        Colors.amber,
+        'Waiting for next run',
+        'Next run in ${_formatSeconds(remaining)}',
+      );
+    }
+    return (Colors.grey, 'Not yet started', null);
+  }
+}
+
+String _formatSeconds(double seconds) {
+  final total = seconds.round().clamp(0, double.maxFinite.toInt());
+  if (total < 60) return '${total}s';
+  final m = total ~/ 60;
+  final s = total % 60;
+  return '${m}m ${s}s';
+}