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/plugins/native-call/NativeCall.podspec

@@ -0,0 +1,23 @@
+require 'json'
+
+package = JSON.parse(File.read(File.join(__dir__, 'package.json')))
+
+Pod::Spec.new do |s|
+  # NOTE: the pod name MUST be 'NativeCall' (PascalCase of the npm package name 'native-call').
+  # Capacitor's `cap sync` writes `pod 'NativeCall', :path => '../../plugins/native-call'` into the
+  # generated Podfile, and CocoaPods then looks for a file literally named NativeCall.podspec whose
+  # s.name is 'NativeCall'. Any other name → "No podspec found for `NativeCall`" and pod install fails.
+  s.name = 'NativeCall'
+  s.version = package['version']
+  s.summary = package['description']
+  s.license = package['license']
+  s.homepage = 'https://bizgaze.com'
+  s.author = 'BizGaze'
+  s.source = { :git => 'https://bizgaze.com/native-call.git', :tag => s.version.to_s }
+  s.source_files = 'ios/Sources/**/*.{swift,h,m}'
+  s.ios.deployment_target = '14.0'
+  s.dependency 'Capacitor'
+  # CallKit + PushKit + AVFoundation are system frameworks (no external pod).
+  s.frameworks = 'CallKit', 'PushKit', 'AVFoundation'
+  s.swift_version = '5.1'
+end