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)

scripts/managers/special_tiles_manager.gd

@@ -153,6 +153,12 @@ func add_powerup_from_item(item_id: int):
 	var effect = get_effect_from_item(item_id)
 	if effect == -1: return
 
+	# VFX: show pickup burst on all peers (mirrors skill VFX pattern)
+	if player.is_multiplayer_authority() and player.has_method("can_rpc") and player.can_rpc():
+		player.rpc("play_skill_vfx", "take_powerup")
+	elif player.has_method("play_skill_vfx"):
+		player.play_skill_vfx("take_powerup")
+
 	# 1-PowerUp Rule: If this is a DIFFERENT power-up, clear the old one
 	var is_different = not inventory.get(effect, false)
 	var already_has_any = false