Collaborative augmented reality (AR) applications are increasingly adopted across diverse fields. In this paper, we introduce ARCanvas, a mobile AR application that enables real-time multiplayer spatial drawing in colocated setting. ARCanvas introduces two key enhancements: 1) its anchor management system that supports stable spatial anchors for persistent shared content, and 2) its networking framework eliminates dependency on commercial cloud services by relying solely on local network infrastructure. The system combines a lightweight networking framework with a collaborative spatial drawing system, optimized for smartphone use. Experimental results demonstrate that ARCanvas maintains consistent performance, with latency increasing linearly with data complexity. Future work aims to evolve ARCanvas from a research prototype into a production-ready platform, making high-quality collaborative AR more accessible on mobile devices.