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)

server/nakama/lua/utils.lua

@@ -3,6 +3,7 @@ local nk = require("nakama")
 local utils = {}
 
 utils.ADMIN_ROLES = { ["admin"] = true, ["moderator"] = true, ["owner"] = true }
+utils.SYSTEM_USER_ID = "00000000-0000-0000-0000-000000000000"
 
 function utils.is_admin(context)
     if not context.user_id then return false end