feat: remember position in timeline on reopen

[franklouwers]

Describe the feature you'd like to request

When re-opening the app, it should open on the most recent post we read last time.

Describe the solution you'd like to see

It currently opens at the most recent one, forcing to scroll down to an unmarked post, then read up again.

Additional information

Also when refreshing the feed, the new posts should load, ideally put a visual marker/separator in place, but remain in place.

[mozzius]

Blocked by bluesky-social/atproto#1834

[fjordBob]

Has a decision already been made about this feature? I think the implementation of this feature would be really great.