FCE-3440: React context for VoIP + useVoIP hook + working example on IOS

[Magmusacy]

Description

Adds a complete iOS VoIP calling example (examples/mobile-client/voip-call/) showing how to wire native PushKit VoIP pushes and CallKit into a Fishjam call, end to end:

• Example app (Expo / React Native):
  • VoipProvider + useVoip context that owns the call lifecycle (available → incoming/connecting → active), driving the Fishjam connection (join on answer/start, leave on end) and the CallKit session. Media (mic/camera) is acquired before joining so tracks publish reliably.
  • Screens matching the fishjam-chat look & feel: Login, Directory, Outgoing call, and an In-call screen with an audio layout and a FaceTime-style video layout (remote full-screen + local PiP).
  • Reusable UI (Avatar, InCallButton, VideoCallView), a shared theme, and login persistence via AsyncStorage with logout.
  • A minimal Deno signaling backend (server/) using certificate-based APNs auth to send VoIP pushes, with setup docs.
• SDK: exports useVoIPEvents (and VoIPEventHandlers / VoipIncomingPayload types) from @fishjam-cloud/mobile-client, so apps can subscribe to PushKit/CallKit events (token registered, incoming, answered, ended) from JS.
• Submodule bump: packages/react-native-webrtc is moved forward to pick up the native PushKit registration + CallKit incoming-call reporting that the example relies on.

Motivation and Context

The mobile SDK already had CallKit primitives, but there was no end-to-end reference for the full native VoIP flow - receiving a PushKit push while backgrounded, presenting the system incoming-call UI via CallKit, and connecting that to a Fishjam room. This PR provides that reference implementation plus the useVoIPEvents bridge and a sample backend, so developers can see and reproduce the whole path.

Documentation impact

• Documentation update required
• Documentation updated in another PR
• No documentation update required

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)

[linear]

FCE-3440