fix: restore viewer state after Quick Look export failure

[Arthur031221]

Reference Issue

Related to #4300. Quick Look cannot export that issue's video texture. This change repairs the viewer after the failed conversion; it does not add video texture support to USDZ.

When a page uses createVideoTexture() and starts Quick Look, USDZExporter rejects because it cannot process the video element. The live model then disappears, its shadow is hidden, a progress activity stays open, and the AR button stays disabled. The cleanup in prepareUSDZ() ran only after a successful export.

Restore the model, shadow visibility, and progress activity when export rejects, then re-enable the AR button. The original error still propagates. After the failed AR attempt, users can keep viewing the model and the AR button is available again; Quick Look still does not open for a video texture.

The browser test loads cube.gltf, assigns lottie-logo.mp4 through the public createVideoTexture() API, and reaches the exporter without a stub. The same error occurred before and after the fix: THREE.USDZExporter: No valid image data found. Unable to process texture.

Before the fix, the AR test printed:

AssertionError: expected [ false, false, false, false ] to deeply equal [ true, true, true, true ]
Chromium: 11 passed, 1 failed, 1 skipped

After the fix, the full Chromium suite passed with 411 passed, 0 failed, 23 skipped. The same suite on the base commit passed twice with 410 passed, 0 failed, 23 skipped. npm run build:tsc -w @google/model-viewer also passes. WebKit and iOS were not run locally because this host lacks WebKit's system libraries.

[diegoteran]

thank you for this