feat(wallpaper): add folder-based wallpaper selection and shuffle - #2990

onFileSelected callback already strips the file:// from path, so this can be removed. We also prefer the Paths.strip helper in general where it is needed

I think we can add a proper function in WallpaperCyclingService, would make it clearer than using dummy.jpg

function cycleFromFolder(screenName, folderPath) {
    listWallpapers(folderPath, files => applyFirstWallpaper(screenName, files));
}

Passing currentWallpaper = "" makes applyCycledWallpaper skip the first image in the folder. With currentPath === "", findIndex returns -1, so currentIndex is coerced to 0 and the non-random branch computes targetIndex = (0 + 1) % length → the second file alphabetically. So selecting a folder with 2+ wallpapers never applies the first one (and with random cycling on, the do/while excludes index 0, so the first image can't be picked on the initial selection either). Only the single-file case works, because 1 % 1 === 0.

Either apply wallpaperList[0] directly for the folder-select path (the applyFirstWallpaper shape suggested earlier), or pass a sentinel that applyCycledWallpaper understands as "start at the first entry" instead of reusing the "next from current" logic.

wallpaperCyclingFolderPath is written here (and declared in SessionData.qml / SessionSpec.js) but never read anywhere — no other QML, and buildServerConfig() doesn't send it to the server either. So the "persist folder selection across restarts" goal isn't actually achieved; after a restart cycling still derives the directory from SessionData.wallpaperPath in cycle(). Either consume it (e.g. prefer it over the current wallpaper's dirname in cycle()) or drop the setting so it isn't persisted dead state.

Select Wallpaper Folder is a new term, but Choose wallpaper folder already exists in quickshell/translations/en.json and is already translated in several locales. CONTRIBUTING asks to reuse existing terms where possible.