feat: redesign Sidebar with hierarchical trees, starring, and collapsible sections

Replace flat app/route/agent lists with expandable tree navigation.
Apps contain their routes and agents hierarchically. Add localStorage-
backed starring with composite keys for uniqueness. Persist expand
state to sessionStorage across page navigations. Add collapsible
section headers, remove button on starred items, and parent app
context labels. Create stub pages for /apps/:id, /agents/:id,
/admin, /api-docs. Consolidate duplicated sidebar data into
shared mock. Widen sidebar from 220px to 260px.

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

src/pages/Admin/Admin.tsx

@@ -0,0 +1,22 @@
+import { AppShell } from '../../design-system/layout/AppShell/AppShell'
+import { Sidebar } from '../../design-system/layout/Sidebar/Sidebar'
+import { TopBar } from '../../design-system/layout/TopBar/TopBar'
+import { EmptyState } from '../../design-system/primitives/EmptyState/EmptyState'
+import { SIDEBAR_APPS } from '../../mocks/sidebar'
+
+export function Admin() {
+  return (
+    <AppShell sidebar={<Sidebar apps={SIDEBAR_APPS} />}>
+      <TopBar
+        breadcrumb={[{ label: 'Admin' }]}
+        environment="PRODUCTION"
+        shift="Day (06:00-18:00)"
+        user={{ name: 'hendrik' }}
+      />
+      <EmptyState
+        title="Admin Panel"
+        description="Admin panel coming soon."
+      />
+    </AppShell>
+  )
+}