Add CenteredPage template and fix SideNav collapse animation

Add CenteredPage template for no-sidebar, centered-content layouts
(login, error, onboarding). Fix SideNav collapse animation where items
slid right — removed nav-level items-center (each item already handles
its own centering) and added overflow-hidden to clip text during the
width transition.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

DESIGN.md

@@ -1092,6 +1092,26 @@ import { DetailPage } from '@/components/templates/DetailPage'
 </DetailPage>
 ```
 
+#### CenteredPage
+
+Full-page layout with no sidebar and centered content. Use for login, sign-up, error pages, onboarding, or any focused single-task flow.
+
+| Prop | Type | Default | Description |
+|------|------|---------|-------------|
+| `topBar` | `ReactNode` | — | Optional TopBar |
+| `maxWidth` | `'sm' \| 'md' \| 'lg' \| 'xl'` | `'md'` | Content max width |
+| `children` | `ReactNode` | — | **Required.** Centered content |
+
+```tsx
+import { CenteredPage } from '@/components/templates/CenteredPage'
+
+<CenteredPage topBar={<TopBar title="" logo={<NswLogo />} />} maxWidth="md">
+  <Card variant="elevated">
+    <CardContent>Login form here</CardContent>
+  </Card>
+</CenteredPage>
+```
+
 ---
 
 ## Do's and Don'ts