wallpaper: a screen can draw from a gallery folder instead of its hearts

Every wallpaper in the gallery is wide, so hearting a whole folder one
thumbnail at a time is the long way to say "this folder". Each screen
now has a source on the engine page: its hearted list, or one gallery
folder picked by chip, stored as the Wallpapers.groups key in Prefs
(folderH, folderV, "" for the hearts).

A folder that has left the gallery gives an empty pool, so that screen
is left alone rather than quietly falling back to its hearts. The
hearted list stays saved behind a folder source and comes back with the
Hearted chip. Switching source needs nothing from the shuffle: draw()
already drops bag entries that are no longer in the pool.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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