feat: ✨ add client and dashboard design system reference pages

[hermannleboss]

Goal

Provide a living design-system reference page in both the client application and the administrator dashboard.

Scope

• Add an authenticated /design-system route to each application.
• Present the established colour, typography, spacing, action, form, status, and data-display patterns using the components already provided by Nuxt UI.
• Make each page reachable from its corresponding sidebar.

Acceptance criteria

• The client page documents and demonstrates its current visual tokens and common UI components.
• The dashboard page documents and demonstrates its current visual tokens and common UI components.
• Both pages reuse the existing UI library and remain responsive and keyboard accessible.
• Navigation exposes the pages without affecting existing application flows.