Refine FlowPathLayer shader uniforms and draw logic

[ibgreen]

Summary

• delegate FlowPathLayer shader configuration to the LineLayer implementation while keeping required modules
• update the FlowPathLayer vertex shader to consume the line and layer uniform blocks and viewport uniforms
• mirror LineLayer.draw so FlowPathLayer populates the line uniform block before issuing draw calls

Testing

• git commit (triggers repository vitest suite)

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https://chatgpt.com/codex/tasks/task_e_6908982623308328a07199c69e827267