[Feature] Incremental offline sync – only download new album assets instead of re-downloading everything on expiry

[PwnTwoFace]

I have searched the existing feature requests to make sure this is not a duplicate request.

• Yes

The feature

First, thanks for Kiosk. I run it as a photo frame for my grandparents so they can see photos the whole family shares into one album, and it's been great for that.

Their situation is why I'm asking: no home internet, only a mobile connection, and mobile data in Germany is expensive. So the device has to be as data-light as possible. Offline mode is otherwise perfect – the frame keeps showing photos even when the connection drops.

The part that doesn't fit is refreshing the album. I saw in the 0.22 notes that on expiry only expired assets are deleted and re-fetched. The issue for my use case is that the whole pool seems to share one expiration, so on expiry everything expires at once and the full album gets re-downloaded – even if only one or two new photos were actually added. For my ~120-photo album that's a full re-download every cycle.

What I'd love: an incremental sync that compares the album against what's already downloaded and only fetches assets that are genuinely new (and ideally removes ones no longer in the album), rather than expiring and re-fetching the whole set.

That would make offline mode viable on metered/expensive connections.