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/lib/providers/settings_provider.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/foundation.dart' show kDebugMode;
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:screen_protector/screen_protector.dart';
 import 'package:sepot/core/backup/backup_service.dart';
@@ -57,14 +58,21 @@ final typingIndicatorsProvider =
         TypingIndicatorsNotifier.new);
 
 /// Zabezpečení obrazovky: blok screenshotů (Android FLAG_SECURE) + skrytí
-/// obsahu v přepínači appek (iOS). Default VYPNUTO — blokuje i vlastní
-/// screenshoty, tak ať se uživatel rozhodne sám.
+/// obsahu v přepínači appek (iOS). Výchozí ZAPNUTO jako u Signalu — stejné
+/// jako nativní default v MainActivity, takže přepínač už neukazuje
+/// „vypnuto" u obrazovky, která chráněná je.
 class ScreenSecurityNotifier extends AsyncNotifier<bool> {
   static const key = 'screen_security_enabled';
+  // Debug build bez ochrany: QA na emulátoru potřebuje screenshoty.
+  static const vychozi = !kDebugMode;
 
   @override
   Future<bool> build() async {
-    final v = (await ref.read(storageProvider).getSetting<bool>(key)) ?? false;
+    final storage = ref.read(storageProvider);
+    // Před odemčením se nastavení přečíst nedá; nic neaplikovat (nativní
+    // default platí) a po unlocku se provider invaliduje.
+    if (!storage.isUnlocked) return vychozi;
+    final v = (await storage.getSetting<bool>(key)) ?? vychozi;
     await _apply(v);
     return v;
   }