feat: take_powerup VFX, rank fix, admin chat management

- Wire take_powerup AnimatedSprite3D on powerup pickup via add_powerup_from_item()
- Make take_powerup animation one-shot (loop: false)
- Fix rank Position label hidden at game start (visible = false, only shows when score > 0)
- Competition ranking for tied scores in main.gd
- Lobby Chat admin tab: system prefix, max messages, wipe, purge old, save config
- Chat Storage admin tab: list/browse/delete individual channel messages
- Backend RPCs: admin_get_chat_config, admin_set_chat_config, admin_purge_old_messages,
  admin_list_channel_messages, admin_delete_channel_message
- Chat config applied on lobby join (max_messages, prefix injection)

scenes/ui/lobby_chat.gd

@@ -9,6 +9,7 @@ var _chat_messages: Array = []
 var _active_chat_context: String = "global"
 var _dm_tabs: Dictionary = {}
 var _dm_messages: Dictionary = {}
+var _chat_config: Dictionary = {"prefix": "", "max_messages": 50, "max_age_days": 0}
 
 func _init(p_lobby: Control):
 	lobby = p_lobby
@@ -40,17 +41,34 @@ func join_global_chat() -> void:
 	
 	_chat_channel = result
 	print("[Chat] Joined global channel: ", _chat_channel.id)
-	
+
 	if not socket.received_channel_message.is_connected(_on_chat_message_received):
 		socket.received_channel_message.connect(_on_chat_message_received)
-	
+
+	# Fetch admin chat config (prefix, max_messages, etc.)
+	if BackendService.has_method("admin_get_chat_config"):
+		var cfg_res = await BackendService.admin_get_chat_config()
+		if cfg_res.has("config"):
+			_chat_config = cfg_res["config"]
+
 	_chat_messages.clear()
-	var history_result = await NakamaManager.client.list_channel_messages_async(NakamaManager.session, _chat_channel.id, 50, false)
+	var limit: int = _chat_config.get("max_messages", 50)
+	var history_result = await NakamaManager.client.list_channel_messages_async(NakamaManager.session, _chat_channel.id, limit, false)
 	if not history_result.is_exception() and history_result.messages:
 		var msgs = history_result.messages.duplicate()
 		msgs.reverse()
 		for msg in msgs:
 			_add_chat_message(msg, false)
+
+	# Inject admin system prefix if configured
+	var prefix: String = _chat_config.get("prefix", "")
+	if not prefix.is_empty():
+		_chat_messages.insert(0, {
+			"sender": "SYSTEM",
+			"content": prefix,
+			"ts": _get_local_time(),
+			"date": Time.get_date_string_from_system()
+		})
 			
 	_trim_old_messages()
 	_refresh_chat_display()