Support macOS 15.6

[PaulSolt]

Can you add support for macOS Sequoia (15.6+)?

I haven't updated to macOS 26 (I don't want to) and would like to use without editing the package and code files.

SkillSplitView.swift

            if #available(macOS 26.0, *) {
                ToolbarSpacer(.fixed)
            }
        }

        ToolbarItem(id: "open") {
            openFolderItem
        }

        if #available(macOS 26.0, *) {
            ToolbarSpacer(.fixed)
        }

[mergesort]

+1 to this. I haven't upgraded yet for other reasons, but would love to use this!