Rendering performance overhaul

What changed

• Moved the minimap to a dedicated fixed canvas with a controlled update rate.
• Moved notifications and loading status to lightweight DOM elements.
• Gave performance diagnostics a canvas that exists only while visible.
• Kept the full-screen overlay as a small input surface rather than a large drawing buffer.
• Reduced transparent texture clearing and compositing in the hot render loop.

The result is lower mobile overhead without reintroducing a 2D fallback for the Three.js world.