Sort library by date of the oldest unread chapter

[meee-low]

Describe your suggested feature

I would like to be able to sort books by the date of the oldest unread chapter. This would allow for an easy way of reading releases chronologically. It would also help with seeing what books I'm the most behind on.

This would be specially helpful for stories that spread across many titles, such as many super-hero crossover events. This would alleviate feature requests like #1106 with a (probably) simpler solution and less niche feature. Outside of that context, it would be generally helpful to see which books you've neglected, as well as keeping books you always read on release at the top/bottom (similar to the latest chapter sort).

Example:

(assume we're on day 30)
Book A has unread chapters released on days 1, 8 and 15 and a read chapter released on day 1.
Book B has unread chapters released on days 10 and 20.

The books should initially be sorted (if ascending) as:

1. Book A (8, 15, ignoring day 1)
2. Book B (10, 20)

After I read that day 8 chapter, update to:

1. Book B (10, 20)
2. Book A (15, ignoring day 1 and 8)

And so on.

Other details

We can currently sort by date fetched, latest chapter and number of unread chapters (among many others). These current options are not enough because:

1. Sorting by number of unread chapters doesn't tell you much if they have wildly different release schedules. I could be behind 3 chapters on a weekly title while a monthly title is only behind by 1.
2. Sorting by latest chapter tells you nothing about the order you are supposed to read them if you have more than 1 unread chapter.
3. Date fetched and latest chapter almost work as a replacement for this, if the user doesn't update the titles and fetch the new chapters. After that is done (or if the chapters have already been released when first fetched), that doesn't work, and it can't be reversed.

Acknowledgements

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