Build a single self-contained HTML file that renders a beautiful 3D nebula / spiral galaxy made of glowing particles. Requirements: thousands of additively-blended glowing point particles arranged in a spiral-galaxy or nebula shape, with a tasteful color gradient from the bright core out to cooler edges, rendered in WebGL; a slow auto-orbit that the user can toggle on/off; full mouse orbit controls (drag to rotate, scroll to zoom) and a "recenter" button that returns the camera to a good default; a control to change the glow / particle size; and a dark starfield background. Crucially, it must look gorgeous with its DEFAULT settings — correctly exposed, not blown-out white or invisible — so it reads as a polished, finished scene the moment it loads. You may load Three.js from a CDN. Return ONLY a single complete self-contained HTML file, no markdown fences, no explanation.
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