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/services/backend_service.gd

@@ -121,9 +121,18 @@ func api_rpc_async(rpc_id: String, payload: String = "{}") -> Dictionary:
 			}
 		
 		# Success
+		var payload_str: String = result.payload if result.payload != null else "{}"
+		if payload_str.is_empty():
+			payload_str = "{}"
+		
+		var parsed_data = JSON.parse_string(payload_str)
+		if parsed_data == null:
+			parsed_data = {}
+			
 		return {
 			"success": true,
-			"payload": result.payload
+			"payload": payload_str,
+			"data": parsed_data
 		}
 		
 	return { "success": false, "error": ErrorCode.NETWORK_ERROR, "message": "Max retries exceeded" }