Fix viewer crash in unsupported GPU contexts

[anton-pascal]

Summary\n- detect browsers/environments with neither WebGPU nor WebGL before mounting the R3F Canvas\n- render a small unsupported 3D viewer fallback instead of initializing Three's WebGPU renderer\n- switch to the same fallback when renderer init still rejects, avoiding repeated failed mounts\n\n## Validation\n- bun run --filter @pascal-app/viewer build\n- bunx biome lint packages/viewer/src/components/viewer/index.tsx\n\nFixes Sentry MONOREPO-EDITOR-59.

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Project	Status	Preview	Updated (UTC)
pascal	🔴 Failed	–	Jun 14, 2026, 4:08 AM

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[anton-pascal]

Nightly Sentry triage (06-16): EDITOR-59 (getSupportedExtensions of null in three.webgpu WebGLBackend.init) still unresolved, last seen 06-16 02:21 on HeadlessChrome/no-GPU clients. Related family: EDITOR-3P Cannot read properties of null (reading 'addEventListener') in @react-three/fiber events connect (last seen 06-16 01:35, Windows prod) — same root cause (missing/torn-down GL context / canvas). Please confirm this PR's guard also covers the R3F connect() null-domElement path, or it may need a companion guard.

[anton-pascal]

Nightly Sentry triage 2026-06-17: EDITOR-59 (getSupportedExtensions on null GL context) is still recurring — latest event 2026-06-17T03:01Z on Mobile Safari iOS 18.7 at editor.pascal.app. Sibling issue EDITOR-AY ("Error creating WebGL context") is the same root cause (null/failed context on device). This PR's unsupported-GPU-context guard should resolve both; bumping for review. No duplicate PR opened.