feat: add WebRTC voice/video call infrastructure

Implements P2P calling over Nostr relays using WebRTC for media
transport and NIP-59 Gift Wraps for encrypted signaling. No custom
server required — only public STUN servers for NAT traversal.

Protocol layer (quartz/nip100WebRtcCalls):
- 6 new event kinds (25050-25055): offer, answer, ICE candidate,
  hangup, reject, renegotiate
- WebRtcCallFactory for creating and gift-wrapping signaling events
- CallIdTag and CallTypeTag for event metadata
- Events registered in EventFactory

Call state machine (commons/call):
- CallState sealed interface with full lifecycle states
- CallManager orchestrating signaling and state transitions
- Follow-gate spam prevention: only followed users can ring,
  non-follows are silently ignored

Android WebRTC integration (amethyst/service/call):
- WebRtcCallSession wrapping Google WebRTC PeerConnection
- CallForegroundService for keeping calls alive in background
- IceServerConfig with default public STUN servers
- User-configurable TURN server support

Android UI (amethyst/ui/call):
- CallScreen with offering, connecting, connected, and ended states
- IncomingCallUI with accept/reject buttons
- ConnectedCallUI with mute, video toggle, speaker, and timer
- Call button added to 1-on-1 DM chat header
- ActiveCall route added to navigation

https://claude.ai/code/session_017hZm7yu7CzmcQgZGSaqSXS

gradle/libs.versions.toml

@@ -48,6 +48,7 @@ secp256k1KmpJniAndroid = "0.23.0"
 securityCryptoKtx = "1.1.0"
 slf4j = "2.0.17"
 spotless = "8.4.0"
+streamWebrtcAndroid = "1.3.8"
 tarsosdsp = "2.5"
 translate = "17.0.3"
 jetbrainsCompose = "1.10.3"
@@ -157,6 +158,7 @@ okhttpCoroutines = { group = "com.squareup.okhttp3", name = "okhttp-coroutines",
 secp256k1-kmp-common = { group = "fr.acinq.secp256k1", name = "secp256k1-kmp", version.ref = "secp256k1KmpJniAndroid" }
 secp256k1-kmp-jni-android = { group = "fr.acinq.secp256k1", name = "secp256k1-kmp-jni-android", version.ref = "secp256k1KmpJniAndroid" }
 secp256k1-kmp-jni-jvm = { group = "fr.acinq.secp256k1", name = "secp256k1-kmp-jni-jvm", version.ref = "secp256k1KmpJniAndroid" }
+stream-webrtc-android = { group = "io.getstream", name = "stream-webrtc-android", version.ref = "streamWebrtcAndroid" }
 tarsosdsp = { group = "be.tarsos.dsp", name = "core", version.ref = "tarsosdsp" }
 unifiedpush = { group = "com.github.UnifiedPush", name = "android-connector", version.ref = "unifiedpush" }
 vico-charts-compose = { group = "com.patrykandpatrick.vico", name = "compose", version.ref = "vico-charts-compose" }