Add a Contrast stepper to the reader View menu for fixed-layout
(PDF/CBZ) documents. It increases and decreases page contrast via a
CSS filter on the rendered page images, applies to the whole book,
and is stored per-book (local to the current document).
The filter is built in applyFixedlayoutStyles by combining any
dark-mode invert with the contrast amount into a single filter
declaration. Persisted with skipGlobal so it never touches global
view settings, and added to FoliateViewer's effect dependencies so
the change re-applies across all rendered pages.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Huang Xin
0b4993407c