background fetch

### Problem I have

partly due to the message overlaps that tend to happen when fetching a catchup I wait till closer to the minute that the last one was fetched (and then loose sense of time and wait for another hour), also partly due to sometimes i don't get around to catching up in time to end up exceeding the message limit(which I've seen can happen due to hashtags spamming something or just time) my instance allows for I thought of the below.

### Solution I'd like

would be nice if the catchup feature had a background fetch where when in that mode it just grabs messages from your feed as the server notifies of new messages (assuming fedi instance supports that) and just runs until user clicks a finish button which would do any final fetching required and then display it.
I'm thinking at least when the mode is started it should start by fetching up to the last catchup stored

### Alternatives considered

_No response_

### Other

it likely should save progress once in a while in this mode as I believe chrome at least can unload a tab automatically, and i suppose on mobile the tab being pushed out of memory is likely to happen often.