Native calls: fix answered-call drop, multi-device ring, add in-app call screen

Root cause of "answered but the call disconnects": native calls carry media
over LiveKit and bypass the mesh, so the server only learned "answered" from a
WebView POST. On a cold/locked answer the app is still launching and that event
was lost, so the 40s unanswered timer fired and cancelled the live call. Fix:
- Plugin: notifyListeners("answerCall", retainUntilConsumed:true) so a killed/
  locked pickup isn't lost before the WebView JS attaches.
- Server markDmAnswered: emit call-taken to the callee's OTHER devices (stop the
  ring; no teardown) and call-answered to the caller (flip UI to connected).
- Server declineDmCall: ignore a decline once the call is answered, so dismissing
  a stale ring on a second device can't kill the live call.

Also adds a UI-only in-app call screen for native calls (caller + callee):
avatar, name, live timer, mute (-> plugin), end (-> CallKit). Native media has
no meeting window of its own; this covers "no meeting window / can't unmute".

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

mobile/plugins/native-call/ios/Sources/NativeCallPlugin/NativeCallPlugin.swift

@@ -195,7 +195,10 @@ public class NativeCallPlugin: CAPPlugin, CAPBridgedPlugin, PKPushRegistryDelega
         // NATIVELY so the mic works with the active CallKit call and audio survives backgrounding.
         connectRoom(url: (data["livekitUrl"] as? String) ?? "", token: (data["livekitToken"] as? String) ?? "")
         var ev = data; ev["callUUID"] = uuid.uuidString
-        notifyListeners("answerCall", data: ev)
+        // Answering from a KILLED/locked state launches the app — the WebView's JS listener may not be attached
+        // yet, so retain the event until it is. Without this the "answered" signal is lost, the server's
+        // unanswered timer fires, and the call is cancelled ~40s after pickup.
+        notifyListeners("answerCall", data: ev, retainUntilConsumed: true)
     }
 
     public func provider(_ provider: CXProvider, perform action: CXEndCallAction) {