## Summary
Fix 3D asset disappearing when switching between 3D and image outputs in
app mode — missing `onUnmounted` cleanup leaked WebGL contexts.
## Changes
- **What**: Add `onUnmounted` hook to `Preview3d.vue` that calls
`viewer.cleanup()`, releasing the WebGL context when Vue destroys the
component via its v-if chain. Add unit tests covering init, cleanup on
unmount, and remount behavior.
## Review Focus
When switching outputs in app mode, Vue's v-if chain destroys and
recreates `Preview3d`. Without `onUnmounted` cleanup, the old `Load3d`
instance (WebGL context, RAF loop, ResizeObserver) leaks. After ~8-16
toggles, the browser's WebGL context limit is exhausted and new 3D
viewers silently fail to render.
<!-- Pipeline-Ticket: e36489d2-a9fb-47ca-9e27-88eb3170836b -->
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Co-authored-by: Alexander Brown <drjkl@comfy.org>
