Fix page shift when toggling "offset double spreads" setting

The pages should get shifted to
- the left when enabling
- the right when disabling
the setting.

Currently, the "page push" is inverted.
This can cause the page that is most likely being read to get pushed out of the screen. Which then requires the user to change the page.

Example:

Active page: 1+2
Enable setting: -> 2 (currently) instead of -> 1

CHANGELOG.md

@@ -26,6 +26,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 - (**Library**) Fix total library size chip color in light mode 
 - (**Browse**) Fix missing pinned sources in the source language filter
 - (**Browse**) Fix incorrectly showing "local source" source in the source language filter (the local source can't be disabled)
+- (**Reader**) Fix page shift when toggling the "offset double spreads" setting (currently: enable: shift to the right; disable: shift to the left – now: inverted)
 
 ## [20251230.01] (r2937) - 2025-12-30