MorphViewerPlugin doesn't work properly in `scene_viewer`

## Bevy version and features

0.17.3 and main

## What you did

```sh
cargo run --example scene_viewer --features "free_camera" -- assets/models/animated/MorphStressTest.gltf
```
Then press Space to pause the animation and use keys to control the morph target.

## What went wrong

Expected: The morph target’s weight should change with key presses and remain set.
Actual: When a key is pressed, the morph target weight just changes slightly then is reset immediately, so no visual change can be seen. Also, the Left Shift key seems to be conflicting with the free camera controller.

Run the reported cargo command for the scene_viewer example, load MorphStressTest.gltf, pause with Space, and reproduce the morph-target key controls and Left Shift conflict. Trace the morph animation and free-camera input handling involved in that example; done means key presses visibly change and retain the morph weight without conflicting with camera controls.

Written by the indexing model from the issue text.