Compound component API replacing monolithic Sidebar. DS provides shell (Sidebar, Sidebar.Header, Sidebar.Section, Sidebar.Footer, Sidebar.FooterLink) + standalone SidebarTree and useStarred exports. Application controls all content, icons, sections. Adds icon-rail collapse mode. Breaking change — coordinate with server UI migration. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>