fix: support Windows

[TATP-233]

Summary

• resolve MuJoCo off-screen GL to GLFW on Windows instead of Linux headless OSMesa logic
• route backend playback video export through the shared imageio writer
• add imageio-ffmpeg so playback MP4 export does not require a system ffmpeg install
• update playback tests for Windows GL resolution and shared video writing

Validation

• make test-all
• uv run eval --algo sac --task g1_walk_flat --sim mujoco