Changing `WindowMode` on web crashes because there is no `Monitor` entity

## Bevy version and features

[v0.16.1](https://github.com/bevyengine/bevy/releases/tag/v0.16.1) ~ 53ddd5e615ae15ad796b36724ddd67516bec0c84

## \[Optional\] Relevant system information

```
AdapterInfo { name: "NVIDIA GeForce RTX 4060 Ti", vendor: 0, device: 0, device_type: Other, device_pci_bus_id: "", driver: "", driver_info: "", backend: BrowserWebGpu, subgroup_min_size: 4, subgroup_max_size: 128, transient_saves_memory: false }
GPU clustering is supported on this device.
Some GPU preprocessing are limited on this device.
```

## What you did

Min repro can be found here https://github.com/hukasu/bevy_min_repro/tree/web-window-mode
Ran with `bevy run web` and `bevy run --features="webgpu" web`

## What went wrong

Changing `WindowMode` crashed wasm

## Additional information

Use the linked bevy_min_repro web-window-mode branch first, running it with `bevy run web` and with `bevy run --features="webgpu" web`. Trace the web WindowMode change and its Monitor handling; done means changing WindowMode no longer crashes wasm in either configuration.

Written by the indexing model from the issue text.