chore: separate client preload

[teemingc]

This PR separates the client-side page preloading logic into its own module, which is dynamically imported after the app has started. However, I'm not sure if the trade-off is worth it, or rather, I'm not sure how to measure the impact of this.

Pros:

1. 300 bytes less transfer size if your browser supports data saving mode and its enabled
2. Less code to parse before the page loads?

Cons:

1. 1 KB more transfer size with data saving mode off or unsupported
2. Also, an additional network request for loading the preload code separately

Turns out the transfer size of the client module with the preload code in the same chunk is smaller than having both in separate chunks.

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