Double Page (No Cover) now keep opposite flow of Double Page fixing Split Double Spreads

[kanjieater]

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Hi Gotson 👋 Long time admirer & user, first time contributor - let me know if there's anything I can do to have this move forward, thanks.

Use-Case: Attempting to solve https://github.com/gotson/komga/issues/803. Users who read with double pages want to easily view double spreads where both pages are shown together. In some cases, double spreads are shifted so that only one page of the double spread is visible. The aim is to provide a feature that allows users to read pages together as double spreads.

Double Pages & Double Pages (no cover): This enables users to switch between these layouts to ensure that double spreads are always displayed together. By injecting an empty page after a Landscape page in DP NC layout, Split Double Spreads (SDS) are displayed correctly as double spreads, and users can easily switch to the appropriate layout easily ("D" hotkey).

Example: A Landscape page can throw things off - like this inner leaflet scan after the double cover/back landscape pages:

Then the next page was this: Original Algorithm: Both layouts could be in lockstep after a landscape page disrupted the 2 page stepping This resulted in the flow through the whole volume being off by one, and no way to see it correctly in komga besides manually readjusting potentially thousands of volumes and pages manually:

This PR, Now you can switch to the alternate layout if it matches you book:

Now you can switch to the right flow is as the author intended:

The additional issue of where pages could do: DP: [E, 1], [2, 3], [4, E], [5, E] has now been changed to always just show the next pages next to each other: [E, 1], [2, 3], [4, 5] If the last page was a Split Double Spread, then there was no way to see the two pages together previously. I think this was done under the assumption that the last page was always the back cover, but issues of comics online often don't have this given they haven't been compiled into a volume (which may add additional pages between chapters or an ending cover)

Example of Original Algorithm

This PR:

The last additional issue brought up in 803, was resolved a year or two before (not adding an empty page if the first page was landscape). This should resolve the concerns from 803 completely.