Add high-resolution PNG export for the 3D soundscape canvas

[Copilot]

This adds a downloadable high-quality image export for the 3D soundscape preview. The new export path renders the active Three.js scene off-screen at print-friendly resolution instead of reusing the visible canvas buffer.

• UI surface

  • Adds a downloadHighQualityImage action to the existing fullscreen preview controls alongside package and GLB export.

• Preview export state

  • Captures the live preview canvas, renderer, scene, and camera from the React Three Fiber canvas.
  • Stores those references centrally so export can use the current camera position and scene state without rebuilding the preview separately.

• High-resolution render path

  • Introduces a dedicated canvas export utility that:
    • scales output from screen size to a higher DPI target,
    • renders the current Three.js scene into an off-screen WebGL canvas,
    • preserves renderer color/tone/shadow settings,
    • downloads the result as PNG.

• Output fidelity

  • Applies the preview container background color to the exported image so the PNG matches the fullscreen presentation more closely, rather than exporting a transparent canvas by default.

• Cleanup behavior

  • Clears preview export references on unmount.
  • Defers blob URL revocation to avoid racing browser download initiation.

await exportHighQualityCanvas({
  sourceCanvas: previewCanvas,
  renderer: previewRenderer,
  scene: previewScene,
  camera: previewCamera,
  fileName: `${baseName}.png`,
})

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