Add a "Download on Wi-Fi only" setting #79

What

Book downloads run on any connection. DownloadManager uses NetworkType.CONNECTED, and nothing in the app checks metered networks for downloads.

Bite

A 677 MB book downloads over cellular with no warning. A user queueing a few books from a big library can burn through a data plan without knowing.

Proposed shape

• A setting, on by default, that makes the download work use NetworkType.UNMETERED.
• A one-time override per download for people who want it over cellular right now.
• Say in the UI when downloads are waiting for Wi-Fi.

Why not in the big-library build

It is download policy and a new setting, not library load cost.

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