Fix loadSwiftData fileExists path / URL-string mismatch

[ShoootingStaR693]

SDWallpaperVideo.videoURL / .previewImage are stored as
URL.absoluteString (file://…), but loadSwiftData() checked them
with FileManager.fileExists(atPath:), which expects a plain path.
The check always returned false, so every launch entered the recovery
branch and read the @Attribute(.externalStorage) video / thumbnail
properties. Effects:

1. Application Support fills up with duplicate .mov / .png per launch.
2. Touching @Attribute(.externalStorage) on every launch exposes the
   app to a CoreData NSException (_PFRoutines readBytesForExternalReferenceData:
   → _crashOnException) whenever an external blob is missing or
   unreadable.

Fix: extract URL.path from the stored string before fileExists.