fix: enable video toggle during voice calls

When a call starts as voice-only, no video track exists. Tapping
the camera toggle was calling setVideoEnabled(true) on a null
track, doing nothing.

Now toggleVideo() checks if a video track exists. If not, it
creates one on-demand (addVideoTrack + startCamera), upgrading
the voice call to include video. If toggling off, it stops the
camera to save battery.

https://claude.ai/code/session_017hZm7yu7CzmcQgZGSaqSXS

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

@@ -227,9 +227,20 @@ class CallController(
     }
 
     fun toggleVideo() {
+        val session = webRtcSession ?: return
         val enabled = !_isVideoEnabled.value
         _isVideoEnabled.value = enabled
-        webRtcSession?.setVideoEnabled(enabled)
+
+        if (enabled && session.getLocalVideoTrack() == null) {
+            // Video track doesn't exist yet (voice call upgraded to video)
+            session.addVideoTrack()
+            _localVideoTrack.value = session.getLocalVideoTrack()
+        } else {
+            session.setVideoEnabled(enabled)
+            if (!enabled) {
+                session.stopCamera()
+            }
+        }
     }
 
     fun cycleAudioRoute() {