fix: move ringtone and audio mode setup off the main thread

RingtoneManager.getRingtone() and audio mode switching do disk I/O,
causing StrictMode DiskWriteViolation when called from the state
collector on the main thread. Wrap startRinging() and
switchToCallAudioMode() in withContext(Dispatchers.IO).

https://claude.ai/code/session_01LR8NmFGdMoDTVcfKjL7HWR

amethyst/src/main/java/com/vitorpamplona/amethyst/service/call/CallController.kt

@@ -117,7 +117,7 @@ class CallController(
             callManager.state.collect { state ->
                 when (state) {
                     is CallState.IncomingCall -> {
-                        audioManager.startRinging()
+                        withContext(Dispatchers.IO) { audioManager.startRinging() }
                         showIncomingCallNotification(state.callerPubKey)
                     }
 
@@ -128,7 +128,7 @@ class CallController(
                     is CallState.Connecting -> {
                         audioManager.stopRinging()
                         audioManager.stopRingbackTone()
-                        audioManager.switchToCallAudioMode()
+                        withContext(Dispatchers.IO) { audioManager.switchToCallAudioMode() }
                         audioManager.acquireProximityWakeLock()
                         NotificationUtils.cancelCallNotification(context)
                     }