Add routing, an admin guard, and an empty state

[Kaushik4141]

The app has no router today. App.tsx:97-102 treats any non-/ path segment as an item slug, so /admin would be looked up as an item literally named "admin" — the single easiest thing to get wrong in this work.

• Add react-router-dom and lift the existing screen-state machine into routes, keeping /:slug as the QR deep-link
• Make the slug branch skip reserved prefixes
• Add an <AdminRoute> guard backed by GET /api/v1/user/me

Empty state

• With zero items seeded, ProgressScreen renders a zero-length grid and the flow silently dead-ends — which is exactly what every fresh deployer sees first. Show "No {itemNamePlural} have been added yet", linking to /admin when the viewer is an admin.

Blocked by the admin API issue.