fix(network): prevent null multiplayer peer crashes offline/teardown and fix Nakama RPC session validation

scripts/managers/player_action_manager.gd

@@ -22,7 +22,7 @@ func after_action_completed():
 		return
 	player._is_processing_action = true
 	
-	if player.is_multiplayer_authority():
+	if player.can_rpc() and multiplayer.has_multiplayer_peer():
 		player.update_finish_availability()
 	
 	# Clear the highlights after placing the tiles. (Quickfix for Clientside)
@@ -30,12 +30,12 @@ func after_action_completed():
 	
 	# Only update UI if this is the LOCAL HUMAN PLAYER
 	# Guard against stale callbacks after peer teardown (host quitting solo match)
-	if not multiplayer.has_multiplayer_peer():
+	if not multiplayer or not multiplayer.has_multiplayer_peer():
 		player._is_processing_action = false
 		return
 	if multiplayer.get_unique_id() == player.get_multiplayer_authority():
 		# Sync playerboard (Bots DO need to sync their board logic, just not update local UI)
-		if player.is_multiplayer_authority() and player.has_method("can_rpc") and player.can_rpc():
+		if player.can_rpc() and multiplayer.has_multiplayer_peer():
 			var main = player.get_tree().get_root().get_node_or_null("Main")
 			if main:
 				main.rpc("sync_playerboard", player.name.to_int(), player.playerboard)