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/core/crypto/sepot_crypto.dart

@@ -81,6 +81,32 @@ class SepotSealedMessage {
 }
 
 /// Jeden one-time prekey k publikaci na relay.
+/// Nový signed prekey k nahrání na relay (rotace).
+class SepotSignedPreKey {
+  SepotSignedPreKey({
+    required this.id,
+    required this.publicKey,
+    required this.signature,
+  });
+
+  final int id;
+  final Uint8List publicKey;
+  final Uint8List signature;
+}
+
+/// Nový last-resort Kyber prekey k nahrání na relay.
+class SepotKyberPreKey {
+  SepotKyberPreKey({
+    required this.id,
+    required this.publicKey,
+    required this.signature,
+  });
+
+  final int id;
+  final Uint8List publicKey;
+  final Uint8List signature;
+}
+
 class SepotOneTimePreKey {
   SepotOneTimePreKey({required this.id, required this.publicKey});
 
@@ -420,4 +446,64 @@ class SepotCrypto {
     }
     return result;
   }
+
+  /// Nový signed prekey (rotace). Starý zůstává ve store — relace založené
+  /// z bundle těsně před rotací ho ještě potřebují.
+  Future<SepotSignedPreKey> generateSignedPreKey({
+    required int id,
+    required int timestampMs,
+  }) async {
+    final identityPrivate = PrivateKey.deserialize(
+      bytes: _identity.privateKey.toList(),
+    );
+    final private = PrivateKey.generate();
+    final public = private.getPublicKey();
+    final signature = identityPrivate.sign(
+      message: public.serialize().toList(),
+    );
+    await _stores.signedPreKey.storeSignedPreKey(
+      id,
+      SignedPreKeyRecord(
+        id: id,
+        publicKey: public,
+        privateKey: private,
+        signature: signature.toList(),
+        timestamp: BigInt.from(timestampMs),
+      ),
+    );
+    return SepotSignedPreKey(
+      id: id,
+      publicKey: Uint8List.fromList(public.serialize().toList()),
+      signature: Uint8List.fromList(signature.toList()),
+    );
+  }
+
+  /// Nový last-resort Kyber prekey (rotace PQXDH kotvy).
+  Future<SepotKyberPreKey> generateKyberPreKey({
+    required int id,
+    required int timestampMs,
+  }) async {
+    final identityPrivate = PrivateKey.deserialize(
+      bytes: _identity.privateKey.toList(),
+    );
+    final keyPair = KyberKeyPair.generate();
+    final public = keyPair.getPublicKey();
+    final signature = identityPrivate.sign(
+      message: public.serialize().toList(),
+    );
+    await _stores.kyberPreKey.storeKyberPreKey(
+      id,
+      KyberPreKeyRecord.create(
+        id: id,
+        keyPair: keyPair,
+        signature: signature.toList(),
+        timestamp: BigInt.from(timestampMs),
+      ),
+    );
+    return SepotKyberPreKey(
+      id: id,
+      publicKey: Uint8List.fromList(public.serialize().toList()),
+      signature: Uint8List.fromList(signature.toList()),
+    );
+  }
 }