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/utils/oott_api.dart

@@ -7,6 +7,7 @@ import 'package:frontend/utils/pref_utils.dart';
 import '../model/arp_scanner_status.dart';
 import '../model/device.dart';
 import '../model/device_event.dart';
+import '../model/device_summary.dart';
 import '../model/device_type.dart';
 import '../model/notification.dart';
 
@@ -157,6 +158,13 @@ class BackendAPI {
         .toList();
   }
 
+  Future<DeviceSummary> getDeviceSummary() async {
+    debugPrint('About to call GET /devices/summary');
+    final response = await _dio.get('/devices/summary');
+    debugPrint('Received: ${response.data}');
+    return DeviceSummary.fromJson(response.data as Map<String, dynamic>);
+  }
+
   Future<ArpScannerStatus> getArpScannerStatus() async {
     debugPrint('About to call GET /arp_scanner/status');
     final response = await _dio.get('/arp_scanner/status');