Pre-macOS 26 doesn't reliably surface a detail-pane NavigationStack's
toolbar confirmation items in the window title bar, leaving the
"Add Source" actions invisible. Gate the toolbar placement behind
macOS 26+ and fall back to an inline form/header button on older
macOS across the add-source views and the sources list.
Use grouped Form style with LabeledContent rows and move primary
actions into the sheet toolbar for SMB, WebDAV, Local Folder, Remote
Server and the Edit sheet. iOS and tvOS branches unchanged.
Add a TVSourceRowLabelStyle for consistent icon/text spacing, switch
the Scan Network button to TVFormRowButtonStyle so it matches the
NavigationLink rows, and drop the duplicate navigationTitle in
AddWebDAV/AddSMB views since the title is already shown in the
TVSidebarDetailContainer sidebar.
