Add an Appearance sub-section to the Slint settings, mirroring Tauri's:
theme + auto-theme, font/UI-scale/language, startup page, sidebar playlist
collage (wired to the sidebar flag), notifications/toasts/tray, title bar +
window controls, immersive + miniplayer. Controls reuse QbzToggle/QbzSelect
and the AudioSettings/PlaybackSettings layout. Most controls are visual
stubs (AppearanceState + TODO callbacks) pending backend wiring.
cc