Give TVSidebarDetailContainer an optional bottom action slot and use it to
show the Add Source button beside the sources list on tvOS. Switch the
Settings > Sources list from a focus-capturing List to the same
ScrollView+LazyVStack layout MediaSourcesView already uses, drop
.buttonStyle(.card) so row icons no longer clip, and bump the row
icon-to-title spacing to 24pt. Replace the sheet-based Add/Edit flow in
MediaSourcesView with navigationDestinations wrapped in the sidebar
container, and decorate each Add Source form (WebDAV, SMB, remote server,
PeerTube browse) with its own sidebar icon and title.
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