Introduces a small persistent appearance contract: WallpaperTheme
(Dawn/Violet/Mint/Slate), ThemeMode (System/Light/Dark), reduce_motion.
Defaults to Dawn + System + motion-on. Field accessors and setters live
on the struct so the core can mutate without exposing internals; serde
support uses kebab-case so the on-disk form matches the design CSS
naming.
Tests cover defaults, mutation independence, and a JSON round-trip with
the kebab-case names. serde_json is added as a dev-dependency only —
the production crate does not depend on it.
ZacharyZhang-NY