[功能]: 壁纸库与轮换 / Wallpaper library and rotation

The current state of Dream Skin's mental model is "one active theme plus a list of saved themes". Wallpapers are not independent objects - they are packaged with the theme package (background image + `theme.json` + `theme.css` three required payloads), imported and switched together. There is a "change background image" option in the tray/menu bar, but it is a **local replacement of the current theme's background image** (from version 1.5.16, the current `theme.json`, colors, layout parameters, and verified `theme.css` will be retained), not from a wallpaper library.