fix(app): secrets are kept out of screenshots and the recent apps

The recovery phrase, recovery codes and an authenticator's QR code and
key now show inside a SecretScreen: on Android it holds FLAG_SECURE while
it shows (through a channel each app's MainActivity answers, counting
holds, as several may show at once; ScreenGuard.kt), so screenshots,
screen recordings and the recents view don't get them; on iOS, which has
no such flag, it covers them while the app isn't in front, so the app
switcher's picture doesn't hold them (Dart-side, not checked on a device).
Desktops do neither: a window loses focus whenever its person turns to a
password manager. Todo's debug APK builds with it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/test/password/widget/secret_screen_test.dart

@@ -0,0 +1,110 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart';
+import 'package:halkyon_app/testing.dart';
+import 'package:material_ui/material_ui.dart';
+
+void main() {
+  Future<void> pump(WidgetTester tester, Widget child) => tester.pumpWidget(
+    MaterialApp(
+      localizationsDelegates: testLocalizationsDelegates,
+      supportedLocales: HalkyonAppLocalizations.supportedLocales,
+      home: Scaffold(body: SingleChildScrollView(child: child)),
+    ),
+  );
+
+  testWidgets('keeps the screen out of screenshots and recent apps while a '
+      'secret shows, and lets it go after', (tester) async {
+    final guard = FakeScreenGuard();
+    await pump(
+      tester,
+      SecretScreen(guard: guard, child: const Text('the phrase')),
+    );
+    expect(guard.held, 1);
+
+    await pump(tester, const Text('something else'));
+    expect(guard.held, 0);
+  });
+
+  testWidgets('covers the secret while the app is not in front, where asked '
+      '(iOS, whose app switcher shows a picture of it)', (tester) async {
+    await pump(
+      tester,
+      SecretScreen(
+        guard: FakeScreenGuard(),
+        coversWhenAway: true,
+        child: const Text('the phrase'),
+      ),
+    );
+    expect(find.byKey(SecretScreen.coverKey), findsNothing);
+
+    tester.binding.handleAppLifecycleStateChanged(AppLifecycleState.inactive);
+    await tester.pump();
+    expect(find.byKey(SecretScreen.coverKey), findsOneWidget);
+
+    tester.binding.handleAppLifecycleStateChanged(AppLifecycleState.resumed);
+    await tester.pump();
+    expect(find.byKey(SecretScreen.coverKey), findsNothing);
+  });
+
+  testWidgets('the phrase, the recovery codes and an authenticator are each '
+      'shown as a secret', (tester) async {
+    final code = TextEditingController();
+    addTearDown(code.dispose);
+    for (final view in [
+      const RecoveryPhraseView(
+        phrase:
+            'abandon abandon abandon abandon abandon abandon abandon abandon '
+            'abandon abandon abandon abandon abandon abandon abandon abandon '
+            'abandon abandon abandon abandon abandon abandon abandon art',
+      ),
+      const RecoveryCodesView(codes: ['AAAA-BBBB']),
+      AuthenticatorSetupView(
+        setup: const AuthenticatorSetup(
+          id: 'a',
+          uri: 'otpauth://totp/x:anna?secret=JBSWY3DPEHPK3PXP',
+        ),
+        code: code,
+      ),
+    ]) {
+      await tester.pumpWidget(
+        AppScope(
+          dependencies: TestAppDependencies(gateway: FakeAuthGateway()),
+          child: MaterialApp(
+            localizationsDelegates: testLocalizationsDelegates,
+            supportedLocales: HalkyonAppLocalizations.supportedLocales,
+            home: Scaffold(body: SingleChildScrollView(child: view)),
+          ),
+        ),
+      );
+      expect(find.byType(SecretScreen), findsOneWidget, reason: '$view');
+    }
+  });
+
+  testWidgets("the platform's guard asks Android's activity, counted", (
+    tester,
+  ) async {
+    final calls = <String>[];
+    tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(
+      PlatformScreenGuard.channel,
+      (call) async {
+        calls.add(call.method);
+        return null;
+      },
+    );
+    addTearDown(
+      () => tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(
+        PlatformScreenGuard.channel,
+        null,
+      ),
+    );
+    const guard = PlatformScreenGuard(android: true);
+    await guard.hold();
+    await guard.release();
+    expect(calls, ['hold', 'release']);
+
+    // Elsewhere, nothing to ask.
+    calls.clear();
+    await const PlatformScreenGuard(android: false).hold();
+    expect(calls, isEmpty);
+  });
+}