feat(ios): native CallKit + PushKit VoIP calling plugin + web bridge

The native call feature (iOS). Backward-compatible: without the plugin (current
builds) nativeCallOn() is false and every CallKit branch is skipped, so web/older
builds behave exactly as before.

Native (mobile/plugins/native-call, a local Capacitor plugin like audio-route):
- PushKit: registers for VoIP pushes, reports the VoIP token to JS (-> /api/v1/devices
  'ios-voip'). On an incoming VoIP push, reports a CallKit incoming call (full-screen
  ring, works when the app is force-killed).
- CallKit: answer/decline/end -> events to JS; configures the call AVAudioSession on
  didActivate so the WebView's WebRTC audio rides a call-priority session (background).
- Outgoing calls register with CallKit too (reportOutgoingCall) so they get the same
  active-call background-audio context.
- NativeCall.podspec (frameworks CallKit/PushKit/AVFoundation); added to mobile deps;
  ios-patch.sh now sets UIBackgroundModes = [audio, voip] (voip required for PushKit).

Web bridge (home.html): setupNativeCall() registers the VoIP token, joins on CallKit
answer, leaves/declines on CallKit end; on CallKit devices the in-app call-invite popup
+ WebAudio ring are suppressed (the system rings instead); outgoing calls are reported
to CallKit; call-end events dismiss the CallKit call. calls.js threads a stable call
uuid through the dm-call/group-call WS events + start responses so both sides can match
the CallKit call.

Needs a Codemagic build to compile the plugin; first on-device iteration expected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

mobile/package.json

@@ -12,6 +12,7 @@
   "dependencies": {
     "audio-route": "file:plugins/audio-route",
     "media-library": "file:plugins/media-library",
+    "native-call": "file:plugins/native-call",
     "share-inbox": "file:plugins/share-inbox",
     "@capacitor-community/safe-area": "^7.0.0",
     "@capacitor/android": "^7.0.0",