Fix AttributeError when provider is temporarily unavailable (#3157)

get_provider() can return None when a provider is temporarily
unavailable (e.g., during reload or network issues). Both
remove_item_from_library() and add_item_to_library() call
.library_edit_supported() on the result without a None check,
causing an AttributeError that surfaces as a 500 error.

Co-authored-by: David Bishop <[email protected]>
Co-authored-by: Claude Opus 4.6 <[email protected]>