fix: improve sidebar layout with scrollable sections and bottom-pinned admin

- Applications section: maxHeight 50vh with scroll overflow
- Starred section: maxHeight 30vh with scroll overflow
- Admin section: pinned to bottom of sidebar via position="bottom"
- Update design-system to 0.1.54 (sidebar section maxHeight, position props)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/LayoutShell.tsx

@@ -754,6 +754,7 @@ function LayoutContent() {
           label="Applications"
           open={appsOpen}
           onToggle={toggleApps}
+          maxHeight="50vh"
         >
           <SidebarTree
             nodes={appTreeNodes}
@@ -775,6 +776,7 @@ function LayoutContent() {
           label="Starred"
           open={starredOpen}
           onToggle={() => setStarredOpen((v) => !v)}
+          maxHeight="30vh"
         >
           <StarredList
             items={starredItems}
@@ -792,6 +794,7 @@ function LayoutContent() {
           open={adminOpen}
           onToggle={toggleAdmin}
           active={isAdminPage}
+          position="bottom"
         >
           <SidebarTree
             nodes={adminTreeNodes}