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/pubspec.lock

@@ -297,6 +297,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.1.5"
+  dart_webrtc:
+    dependency: transitive
+    description:
+      name: dart_webrtc
+      sha256: "078e3c431500147e5cc52b3c6ea41ed538f30c7720cc2467d2186c9251e62716"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.8.2"
   dbus:
     dependency: transitive
     description:
@@ -430,6 +438,14 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  flutter_callkit_incoming:
+    dependency: "direct main"
+    description:
+      name: flutter_callkit_incoming
+      sha256: "2d47db2e8d32d333e0352d37b0a4e643f606a54f8a541db17af7c42f18c58318"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.1.6"
   flutter_driver:
     dependency: transitive
     description: flutter
@@ -570,6 +586,14 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  flutter_webrtc:
+    dependency: "direct main"
+    description:
+      name: flutter_webrtc
+      sha256: c1d3674f61b14cbe98b60f0cce396835b1483163e644b8cd26fd3fdd55360fb9
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.6.2+hotfix.3"
   fuchsia_remote_debug_protocol:
     dependency: transitive
     description: flutter
@@ -852,6 +876,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.0.11"
+  logger:
+    dependency: transitive
+    description:
+      name: logger
+      sha256: "2a0dc097e7b01d942475bdd552356db2d0f768b05540bd4b2b53f1840f2239a7"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.8.0"
   logging:
     dependency: transitive
     description:
@@ -1537,6 +1569,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.1.0"
+  webrtc_interface:
+    dependency: transitive
+    description:
+      name: webrtc_interface
+      sha256: c6f100eac5057d9a817a60473126f9828c796d42884d498af4f339c97b21014f
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.5.1"
   win32:
     dependency: transitive
     description: