fix: not-found state flashes briefly during initial load on detail page

[zigzagdev]

Problem

useWorldHeritageDetail initializes state as { data: null, loading: false, error: null }.

On the first render, before useEffect fires load(), the container evaluates:

• isLoading → false
• isError → false
• !item → true → renders "Not found."

Then useEffect runs, sets loading: true, and the spinner appears. This causes a visible flash of the not-found state on every page load.

Root cause

useState initial value in use-world-heritage-detail.ts:

loading: false  // should be: loading: id != null

Fix

Change the initial state to loading: id != null so the first render already reflects that a fetch is in flight when an id is present.