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/player.gd

@@ -928,11 +928,17 @@ func _refresh_player_visuals():
 		if active_character:
 			apply_loadout(active_character)
 
-func update_rank_visuals(rank: int):
+func update_rank_visuals(rank: int, score: int = -1):
 	var pos_label = get_node_or_null("Position")
 	if not pos_label:
 		return
-		
+
+	# Hide rank until the player has actually scored, so the match doesn't
+	# start with everyone showing a position (e.g. all "1st").
+	if score == 0:
+		pos_label.visible = false
+		return
+
 	if rank <= 4:
 		pos_label.visible = true
 		if race_manager: