fix(app): accept incoming calls, restore from recovery phrase, duress session isolation, message loss

Calls: `call` was missing from the known content types, so every offer was
acknowledged and dropped; the call screen column now fills the width (the
avatar jumped left after hang-up); a ring started from a push ends after
12 s when no offer arrives; STUN fallback when no TURN credentials.
Registration: X-Sepot-Identity-Proof lets a new device reclaim an existing
identity; retry backs off from 30 s to 10 min and exposes a conflict flag.
Prekeys: PrekeysLow triggers a batch of 50 one-time prekeys via POST
/v1/keys; the signed prekey rotates weekly, the Kyber prekey monthly.
Messaging: crypto operations run serially (concurrent encrypts derived the
same message key); stop() waits for in-flight deliveries (auto-lock between
decrypt and persist lost messages); disappearing-message cleanup no longer
overwrites fresh messages; a push wake-up reconnects stale connections; the
outbox carries polls and mentions; length limits on wire input.
Storage: a duress session never exposes the real identity or phrase, does
not start networking and cannot touch the keystore; deleting data in a
duress session leaves the real account alone; PIN guessing is throttled
(5 misses, then 30 s doubling up to 15 min); the duress blob shares the salt
so unlock time does not reveal which PIN was entered; iOS keychain items are
this-device only; biometrics are biometric-only; screen security defaults
to on and is applied after unlock; tapjacking filter in MainActivity.
Tests: content types, shared-salt unwrap, duress isolation.

app/test/storage/decoy_isolation_test.dart

@@ -0,0 +1,86 @@
+// Argon2id (64 MiB) ×6 pod zátěží celé sady: víc než výchozích 30 s.
+@Timeout(Duration(minutes: 3))
+library;
+
+import 'dart:io';
+
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:sepot/core/crypto/dek_wrap.dart';
+import 'package:sepot/core/crypto/sepot_crypto.dart';
+import 'package:sepot/core/storage/secure_storage.dart';
+import 'package:sepot/models/identity.dart';
+
+/// Nátlaková (decoy) relace musí být izolovaná od skutečného účtu:
+/// jiná identita, žádná fráze, „smazat" nesáhne na skutečná data,
+/// a zámek režim resetuje. Keystore je nahrazený mockem, DB jde do temp.
+/// Potřebuje nativní libsignal (LocalIdentity) a sqlcipher.
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  late Directory tmp;
+  late SecureStorageService storage;
+
+  setUpAll(() async => SepotCrypto.init());
+
+  setUp(() async {
+    FlutterSecureStorage.setMockInitialValues({});
+    tmp = Directory.systemTemp.createTempSync('sepot_decoy');
+    storage = SecureStorageService(dbPath: '${tmp.path}/app.db');
+    await storage.unlockWithoutPin();
+    await storage.saveIdentity(await LocalIdentity.create('Alice'));
+    await storage.saveSeedPhrase(List.filled(24, 'abandon'));
+    await storage.enablePin('111111');
+    await storage.enableDecoyPin('222222');
+    storage.lock();
+  });
+
+  tearDown(() {
+    storage.lock();
+    tmp.deleteSync(recursive: true);
+  });
+
+  test('decoy PIN otevře izolovanou relaci', () async {
+    await storage.unlockWithPin('222222');
+    expect(storage.isDecoyMode, isTrue);
+    expect(await storage.loadSeedPhrase(), isNull);
+    final shown = await storage.loadIdentity();
+    expect(shown, isNotNull);
+
+    storage.lock();
+    expect(storage.isDecoyMode, isFalse);
+    await storage.unlockWithPin('111111');
+    expect(storage.isDecoyMode, isFalse);
+    final real = await storage.loadIdentity();
+    expect(real!.id, isNot(shown!.id), reason: 'decoy ukazuje jinou identitu');
+    expect(await storage.loadSeedPhrase(), hasLength(24));
+  });
+
+  test('smazání v decoy relaci nechá skutečná data', () async {
+    await storage.unlockWithPin('222222');
+    await storage.deleteAll();
+    expect(File('${tmp.path}/app.db').existsSync(), isTrue);
+    expect(await storage.hasIdentity(), isTrue);
+    expect(await storage.hasPin(), isTrue);
+    await storage.unlockWithPin('111111');
+    expect(storage.isDecoyMode, isFalse);
+  });
+
+  test('změny keystore jsou v decoy relaci zakázané', () async {
+    await storage.unlockWithPin('222222');
+    expect(() => storage.enableBiometrics(), throwsStateError);
+    expect(() => storage.disablePin(), throwsStateError);
+    expect(
+        () => storage.changePin(oldPin: '222222', newPin: '333333'),
+        throwsStateError);
+  });
+
+  test('po pěti špatných PINech se zámek na čas zavře', () async {
+    for (var i = 0; i < 5; i++) {
+      await expectLater(storage.unlockWithPin('000000'),
+          throwsA(isA<WrongPinException>()));
+    }
+    expect(await storage.pinLockRemaining(), isNotNull);
+    await expectLater(storage.unlockWithPin('111111'),
+        throwsA(isA<PinLockedException>()));
+  });
+}