feat(app): 1:1 voice and video calls

End-to-end encrypted calls between two contacts over WebRTC. Call
signalling (offer, answer, hang-up) travels as ordinary Signal-encrypted
messages through the relay; media is DTLS-SRTP, peer to peer when
possible and through the operator's TURN server otherwise.

- Non-trickle ICE (candidates gathered for up to 3 s), 45 s ring
  timeout, busy handling, and a call log entry in the conversation.
- The system call UI rings even when the app is closed: CallKit from a
  PushKit VoIP push on iOS, a full-screen incoming call from an FCM
  wake-up on Android. The wake-up carries no caller; the app learns who
  is calling from the queued offer once it is unlocked.
- New permissions: camera for video, audio routing and Bluetooth
  headsets, network state; iOS background modes audio and voip.

Push on Android: a fresh install now picks UnifiedPush when a
distributor is installed and FCM otherwise, instead of leaving wake-ups
off. Without a wake-up Android puts the app to sleep and messages wait
until it is opened. Users can switch at any time in Settings.

app/ios/Runner/it.lproj/InfoPlist.strings

@@ -1,5 +1,5 @@
 /* Šepot — testi delle autorizzazioni iOS (italiano) */
-"NSCameraUsageDescription" = "La fotocamera serve a scansionare i codici QR dei contatti e a scattare foto per gli allegati.";
+"NSCameraUsageDescription" = "La fotocamera serve a scansionare i codici QR dei contatti, a scattare foto per gli allegati e per le videochiamate.";
 "NSPhotoLibraryUsageDescription" = "La libreria foto serve a scegliere le foto da inviare in una conversazione.";
 "NSFaceIDUsageDescription" = "Face ID sblocca l'app al posto del PIN.";
-"NSMicrophoneUsageDescription" = "Šepot ha bisogno del microfono per registrare i messaggi vocali.";
+"NSMicrophoneUsageDescription" = "Šepot ha bisogno del microfono per i messaggi vocali e le chiamate.";