fix: scroll list is non-scrollable (#16540)

# Objective

Run `testbed_ui` example:

```
cargo run --example testbed_ui
```

The scroll list is non-scrollable because it's blocked by the front
four-icon node.

## Solution

Add `PickingBehavior::IGNORE` for the front node

## Testing

- Did you test these changes? If so, how?

Yes.

- Are there any parts that need more testing?

No, I guess.

- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
```
cargo run --example testbed_ui
```
- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?

macOS.

examples/testbed/ui.rs

@@ -324,6 +324,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
                     padding: UiRect::all(Val::Px(10.)),
                     ..default()
                 })
+                .insert(PickingBehavior::IGNORE)
                 .with_children(|parent| {
                     for (flip_x, flip_y) in
                         [(false, false), (false, true), (true, true), (true, false)]