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/l10n/generated/app_localizations.dart

@@ -1994,6 +1994,12 @@ abstract class AppLocalizations {
   /// **'Chyba odemčení'**
   String get pinUnlockError;
 
+  /// Hláška na odemykací obrazovce po sérii špatných PINů; {seconds} = počet sekund do dalšího pokusu
+  ///
+  /// In cs, this message translates to:
+  /// **'Příliš mnoho pokusů. Zkus to za {seconds} s.'**
+  String pinLockedFor(int seconds);
+
   /// Titulek SimpleDialog výběru režimu pushů. Stejný text jako title ListTile v nastavení (ř. 184) → stejný klíč. | Title ListTile v nastavení — stejný text a klíč jako titulek dialogu (ř. 44)
   ///
   /// In cs, this message translates to: