Remove Reservation/Return from header nav (dashboard-only access)

# Why?

**As a** user navigating the app
**I want** a simpler header nav
**So that** I'm not offered redundant top-level Reservation/Return entry points when that functionality is already reachable through the Dashboard.

# What?

Remove the "Reservation"/"Return" links from the nav shell (`(protected)/layout.tsx`) — these flows should only be reachable through the Dashboard (alerts, per-item actions, the cart/checkout flow from #359, etc.), not as standalone header nav items.

Confirm no Playwright spec depends on navigating via these header links directly (`golden-path.spec.ts`/`reservations.spec.ts` likely navigate via `page.goto()` directly to routes rather than clicking nav links — verify before removing).

# Testing

Update/remove any Jest/RTL snapshot or nav-link assertions referencing the removed links.