Fix: Move DiagnosticsOverlay Setup to PreStartup Schedule (#24433)

# Objective

- Fixes #24377 
- One of the regressions was already fixed by #24019 . The other
regression (caused seemingly by merely adding TransformGizmo
functionality) revealed a potential system ordering issue when using the
`DiagnosticsOverlayPlugin`: If one setups `DiagnosticsOverlay`s in the
`Startup` schedule, the `build_overlays` observer could fail silently
because the `DiagnosticsOverlayPlane` (which is also created in the
`Startup` schedule) may not have been created yet.

## Solution

Move the system that sets up the`DiagnosticsOverlayPlane` into the
`PreStartup` schedule.

## Testing

- Copied the “What I did” section in
https://github.com/bevyengine/bevy/issues/24377 (`cargo run --release
--example scene_viewer --features="free_camera bevy_dev_tools"
assets/models/FlightHelmet/FlightHelmet.gltf` after the scene_viewer has
been modified as described). The overlay now appears and works
correctly.