Add a hook for async scene loading

## Problem

PR #110 changes how the app gets scenes. Today, `EditorPage`, `samples.tsx`, and the landing page read the scene list directly from a static array. The new scene registry loads scenes through async calls, not a static array.

## Task

Add a hook, for example `useScenes()`. The hook must:

- Call the scene registry.
- Return a loading state.
- Return the scene list when it loads.
- Return an error state if a provider fails.

## Files to update

Change these files to use the hook instead of the static array:

- `EditorPage`
- `samples.tsx`
- The landing page

## Related

- ADR: `docs/adr/0004-catalogue-provider-architecture.md`
- Issue #102
- PR #110

Start with docs/adr/0004-catalogue-provider-architecture.md and the scene registry introduced by PR #110. Trace how EditorPage, samples.tsx, and the landing page currently read the static scene array, then define the hook's loading, loaded, and error behavior from the registry. Done means all three files use the hook and provider failures are represented.

Written by the indexing model from the issue text.