fix: 3D asset disappears when switching to image output in app mode (#9622)

## 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 -->

---------

Co-authored-by: Alexander Brown <drjkl@comfy.org>

src/renderer/extensions/linearMode/flattenNodeOutput.test.ts

@@ -82,4 +82,71 @@ describe(flattenNodeOutput, () => {
     const result = flattenNodeOutput(['1', output])
     expect(result).toEqual([])
   })
+
+  it('flattens non-standard output keys with ResultItem-like values', () => {
+    const output = makeOutput({
+      a_images: [{ filename: 'before.png', subfolder: '', type: 'output' }],
+      b_images: [{ filename: 'after.png', subfolder: '', type: 'output' }]
+    } as unknown as Partial<NodeExecutionOutput>)
+
+    const result = flattenNodeOutput(['10', output])
+
+    expect(result).toHaveLength(2)
+    expect(result.map((r) => r.filename)).toContain('before.png')
+    expect(result.map((r) => r.filename)).toContain('after.png')
+  })
+
+  it('excludes animated key', () => {
+    const output = makeOutput({
+      images: [{ filename: 'img.png', subfolder: '', type: 'output' }],
+      animated: [true]
+    })
+
+    const result = flattenNodeOutput(['1', output])
+
+    expect(result).toHaveLength(1)
+    expect(result[0].mediaType).toBe('images')
+  })
+
+  it('excludes non-ResultItem array items', () => {
+    const output = {
+      images: [{ filename: 'img.png', subfolder: '', type: 'output' }],
+      custom_data: [{ randomKey: 123 }]
+    } as unknown as NodeExecutionOutput
+
+    const result = flattenNodeOutput(['1', output])
+
+    expect(result).toHaveLength(1)
+    expect(result[0].mediaType).toBe('images')
+  })
+
+  it('accepts items with filename but no subfolder', () => {
+    const output = {
+      images: [
+        { filename: 'valid.png', subfolder: '', type: 'output' },
+        { filename: 'no-subfolder.png' }
+      ]
+    } as unknown as NodeExecutionOutput
+
+    const result = flattenNodeOutput(['1', output])
+
+    expect(result).toHaveLength(2)
+    expect(result[0].filename).toBe('valid.png')
+    expect(result[1].filename).toBe('no-subfolder.png')
+    expect(result[1].subfolder).toBe('')
+  })
+
+  it('excludes items missing filename', () => {
+    const output = {
+      images: [
+        { filename: 'valid.png', subfolder: '', type: 'output' },
+        { subfolder: '', type: 'output' }
+      ]
+    } as unknown as NodeExecutionOutput
+
+    const result = flattenNodeOutput(['1', output])
+
+    expect(result).toHaveLength(1)
+    expect(result[0].filename).toBe('valid.png')
+  })
 })