v1.3 — Bug fixes & HiDPI render correction

fix: bug fixes, HiDPI render correction, async ffprobe, cache sync

- wallpaperconfig: configChanged() now updates cached values before emitting signals
- wallpaperengine: fix enable handler guard, fix play/refreshSource race, fix premature malloc_trim
- wallpaperengine: turnOn() guards against double-call watcher/timer leak
- wallpaperengine: getVideos() now filters by video extension only
- videowallpapermenuscene: ffprobe is now async to avoid freezing the UI thread
- videoproxy: document MpvWidget::setProperty routing to mpv
- rootwidgeteventfilter: replace dynamic_cast with qobject_cast
- mpvwidget: use physical pixels (dpr) for HiDPI displays (fix by @DonkeyKongG3me)
- README: credit @DonkeyKongG3me on Telegram for HiDPI fix