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/main.gd

@@ -2262,12 +2262,22 @@ func _on_leaderboard_updated(sorted_scores: Array):
 	else:
 		sorted_players.sort_custom(func(a, b): return a.score > b.score)
 	
-	# Assign rank
+	# Assign rank. Players sharing a score share a rank (standard competition
+	# ranking), and zero-score players get no rank at all — this prevents the
+	# match from starting with everyone displaying a position.
+	var prev_score = null
+	var prev_rank = 0
 	for i in range(sorted_players.size()):
 		var p_node = sorted_players[i].node
+		var p_score = sorted_players[i].score
 		var rank = i + 1
+		# Tie: reuse the rank of the player above with the same score.
+		if prev_score != null and p_score == prev_score:
+			rank = prev_rank
+		prev_score = p_score
+		prev_rank = rank
 		if p_node.has_method("update_rank_visuals"):
-			p_node.update_rank_visuals(rank)
+			p_node.update_rank_visuals(rank, p_score)
 
 func _on_global_timer_updated(time_remaining: float):
 	"""Update the global match timer display."""