Revise post-loading behaviour

[philpax]

After the loading screen, users are currently taken to the pause menu. This is not a particularly exciting call to action.

We should instead do one of the following:

• Show a start button within the loading screen, which will immediately unpause with the default seed when pressed
• Start showing gameplay immediately, and capture the cursor once Biome is clicked upon

I'm leaning towards the former for a few reasons:

• The model may decohere if left to its own devices for an extended period (e.g. the user leaves Biome running in the background while they wait for it to load)
• Showing gameplay without having input captured would be a special state, and require more significant refactoring
• It may not be intuitive that you need to click to engage with the model