Fix global RPC data parsing in BackendService

- Resolved massive bug where api_rpc_async returned raw payload strings instead of the parsed 'data' Dictionary expected by every system (Gacha, Mail, Leaderboard, Admin Panel, etc.)
- Restored Admin Panel's _rpc helper to use the standard result.get('data') format now that BackendService correctly supplies it.

scripts/ui/admin_panel.gd

@@ -301,15 +301,7 @@ func _rpc(rpc_name: String, payload: Dictionary) -> Dictionary:
 		var err: String = str(result.get("message", "Unknown error"))
 		_set_status(err, CLR_STATUS_ERR)
 		return {"error": err}
-	
-	var payload_str: String = result.get("payload", "{}")
-	if payload_str.is_empty():
-		payload_str = "{}"
-		
-	var parsed = JSON.parse_string(payload_str)
-	if typeof(parsed) == TYPE_DICTIONARY:
-		return parsed
-	return {}
+	return result.get("data", {})
 
 func _set_status(msg: String, color: Color = CLR_DIM) -> void:
 	status_label.text = msg