Wire AppearanceSettings into BrowserCore + snapshot

BrowserCore now owns an AppearanceSettings, exposes it through
BrowserSnapshot.appearance, and offers set_wallpaper_theme,
set_theme_mode, set_reduce_motion, reset_appearance mutators. Defaults
match the domain defaults (Dawn / System / motion-on) so existing
behaviour is unchanged for callers who don't touch appearance.

Integration tests cover initial snapshot defaults, mutation persistence
across snapshots, and reset behaviour.

crates/ely_browser_core/tests/appearance.rs

@@ -0,0 +1,46 @@
+use std::error::Error;
+
+use ely_browser_core::{BrowserCore, InitialBrowserConfig};
+use ely_domain::{ThemeMode, WallpaperTheme};
+
+#[test]
+fn snapshot_starts_with_default_appearance() -> Result<(), Box<dyn Error>> {
+    let core = BrowserCore::new(InitialBrowserConfig::ely_defaults()?)?;
+    let snapshot = core.snapshot()?;
+
+    assert_eq!(snapshot.appearance.wallpaper(), WallpaperTheme::Dawn);
+    assert_eq!(snapshot.appearance.theme_mode(), ThemeMode::System);
+    assert!(!snapshot.appearance.reduce_motion());
+    Ok(())
+}
+
+#[test]
+fn setters_persist_into_subsequent_snapshots() -> Result<(), Box<dyn Error>> {
+    let mut core = BrowserCore::new(InitialBrowserConfig::ely_defaults()?)?;
+
+    core.set_wallpaper_theme(WallpaperTheme::Mint);
+    core.set_theme_mode(ThemeMode::Dark);
+    core.set_reduce_motion(true);
+
+    let snapshot = core.snapshot()?;
+    assert_eq!(snapshot.appearance.wallpaper(), WallpaperTheme::Mint);
+    assert_eq!(snapshot.appearance.theme_mode(), ThemeMode::Dark);
+    assert!(snapshot.appearance.reduce_motion());
+    Ok(())
+}
+
+#[test]
+fn reset_appearance_restores_defaults() -> Result<(), Box<dyn Error>> {
+    let mut core = BrowserCore::new(InitialBrowserConfig::ely_defaults()?)?;
+
+    core.set_wallpaper_theme(WallpaperTheme::Slate);
+    core.set_theme_mode(ThemeMode::Light);
+    core.set_reduce_motion(true);
+    core.reset_appearance();
+
+    let snapshot = core.snapshot()?;
+    assert_eq!(snapshot.appearance.wallpaper(), WallpaperTheme::Dawn);
+    assert_eq!(snapshot.appearance.theme_mode(), ThemeMode::System);
+    assert!(!snapshot.appearance.reduce_motion());
+    Ok(())
+}