Add Archived Worktrees sidebar row with enum-based selection
Refactor sidebar selection to use SidebarSelection enum directly in
state instead of separate selectedWorktreeID and boolean. This
eliminates invalid states and follows swift-navigation best practices.
- Add SidebarSelection.archivedWorktrees case
- Replace selectedWorktreeID stored property with selection enum
- Add computed selectedWorktreeID and isShowingArchivedWorktrees
- Add ArchivedWorktreesDetailView placeholder
- Update all selection write operations in reducer