Add device identification guide dialog for unregistered devices

Adds a "How to identify this device" button next to Register Device that
opens a dialog with personalized clues (vendor/type) and steps to track
down the physical device on the network.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/test/widget/device_identification_guide_test.dart

@@ -0,0 +1,60 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/devices/device_identification_guide.dart';
+import 'package:frontend/model/device.dart';
+
+import '../helpers/fixtures.dart';
+import '../helpers/pump_app.dart';
+
+void main() {
+  Device device({String vendor = 'Acme Corp', String deviceType = 'laptop'}) =>
+      Device.fromJson(deviceJson(vendor: vendor, deviceType: deviceType));
+
+  // Pumps a button that opens the dialog, then taps it so the dialog is shown.
+  Future<void> openDialog(WidgetTester tester, Device d) async {
+    await pumpScreen(
+      tester,
+      Builder(
+        builder: (context) => ElevatedButton(
+          onPressed: () => showDeviceIdentificationDialog(context, d),
+          child: const Text('open'),
+        ),
+      ),
+    );
+    await tester.tap(find.text('open'));
+    await tester.pumpAndSettle();
+  }
+
+  testWidgets('shows every guidance step', (tester) async {
+    await openDialog(tester, device());
+
+    expect(find.text('Identify this device'), findsOneWidget);
+    expect(find.text('Check the manufacturer'), findsOneWidget);
+    expect(find.text('Try the unplug test'), findsOneWidget);
+    expect(find.text('Check your router'), findsOneWidget);
+    expect(find.text('Scan its open ports (advanced)'), findsOneWidget);
+  });
+
+  testWidgets('weaves the vendor and type into the intro', (tester) async {
+    await openDialog(
+      tester,
+      device(vendor: 'Acme Corp', deviceType: 'printer'),
+    );
+
+    expect(
+      find.textContaining('Printer'),
+      findsWidgets,
+      reason: 'the inferred device type should appear in the intro',
+    );
+    expect(find.textContaining('Acme Corp'), findsWidgets);
+  });
+
+  testWidgets('closes when Close is tapped', (tester) async {
+    await openDialog(tester, device());
+
+    await tester.tap(find.text('Close'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Identify this device'), findsNothing);
+  });
+}