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/features/calls/call_screen.dart

@@ -89,9 +89,13 @@ class _CallScreenState extends ConsumerState<CallScreen> {
                       RTCVideoViewObjectFit.RTCVideoViewObjectFitCover,
                 ),
               ),
-            SafeArea(
-              child: Column(
-                children: [
+            // Positioned.fill: bez něj si Stack dá sloupci volné rozměry a ve
+            // fázi „ukončeno" (žádný ovládací řádek přes celou šířku) se sloupec
+            // smrskne na šířku jména a zarovná vlevo nahoru — avatar uskočí.
+            Positioned.fill(
+              child: SafeArea(
+                child: Column(
+                  children: [
                   const SizedBox(height: 48),
                   if (!showVideo) ...[
                     SepotAvatar(
@@ -136,6 +140,7 @@ class _CallScreenState extends ConsumerState<CallScreen> {
                     _InCallControls(call: call, controller: controller),
                   const SizedBox(height: 32),
                 ],
+                ),
               ),
             ),
             if (showVideo && !call.cameraOff)