Adds a capability check and an UnsupportedGpuViewerFallback so the viewer
renders an informative panel instead of crashing on environments that expose
neither WebGPU nor WebGL. Capability detection runs post-mount to stay
hydration-safe, the renderer-init failure path swaps to the fallback, and the
fallback signals scene-readiness so the host editor loader does not hang.
Reworked from #403 (anton-pascal) onto current main; resolves the import
conflict and the SSR/scene-ready gotchas surfaced in review.
Co-authored-by: Open Pascal <open@pascal.app>
5fe0965297