Allow saving bookmarks with authenticated URLs

Bookmarks behind authentication couldn't be saved.
The app didn't allow saving when URL metadata check failed, even
though Linkding accepts these URLs.
The error message still displays to inform users that metadata
couldn't be fetched, but it no longer blocks saving.

lib/screens/bookmarks/provider/bookmark_form.provider.dart

@@ -115,7 +115,12 @@ class BookmarkForm extends _$BookmarkForm {
         state.bookmarkValid = true;
         state.checkBookmarkLoadStatus = LoadStatus.loaded;
       } else {
+        // URL check failed (e.g., authenticated URLs), but if URL format is valid,
+        // allow saving anyway since Linkding accepts these URLs
         state.checkBookmarkLoadStatus = LoadStatus.error;
+        if (state.urlError == null) {
+          state.bookmarkValid = true;
+        }
       }
       ref.notifyListeners();
     }