Redesign Home screen with device summary and scanner status

Adds a responsive Home screen combining the notification list with a
device summary panel and ARP scanner status, visible side-by-side on
wide screens and stacked on narrow ones.

Backend:
- New GET /api/devices/summary endpoint returning registered device
  counts and "seen in last 24h / 7 days" breakdowns by registration
  status
- Migration 07 adds indexes on (is_registered) and (last_seen,
  is_registered) so summary queries use index range scans instead of
  full table scans

Frontend:
- HomeScreen replaces NotificationList, embedding notifications,
  device summary card, and ArpScannerCard in a LayoutBuilder-driven
  two-column (≥700 px) or single-column layout
- ArpScannerCard extracted to a shared public widget reused by both
  HomeScreen and StatusScreen
- Nav rail entry renamed to "Home" with home icon

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

frontend/lib/status/status_screen.dart

@@ -3,6 +3,7 @@ import 'dart:async';
 import 'package:flutter/material.dart';
 import 'package:frontend/model/arp_scanner_status.dart';
 import 'package:frontend/utils/oott_api.dart';
+import 'package:frontend/widgets/arp_scanner_card.dart';
 
 class StatusScreen extends StatefulWidget {
   const StatusScreen({super.key});
@@ -29,7 +30,9 @@ class _StatusScreenState extends State<StatusScreen> {
     );
     _tickTimer = Timer.periodic(
       const Duration(seconds: 1),
-      (_) { if (mounted) setState(() {}); },
+      (_) {
+        if (mounted) setState(() {});
+      },
     );
   }
 
@@ -66,100 +69,13 @@ class _StatusScreenState extends State<StatusScreen> {
       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,
-          ),
+        ArpScannerCard(
+          status: _status,
+          statusReceivedAt: _statusReceivedAt,
+          error: _error,
+          isLoading: _isLoading,
+        ),
       ],
     );
   }
 }
-
-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';
-}