feat: add loading and error fallbacks

[arpittkhandelwal]

Summary

This PR prevents the blank editor container shown while the CDN embed script and editor instance are initializing, and gives host applications control over the loading and initialization-error experience.

Closes #82

What changed

• Adds loadingFallback?: ReactNode for a custom loading spinner, skeleton, image, or branded UI.
• Adds errorFallback?: ReactNode | ((error, retry) => ReactNode) for a custom initialization-error experience.
• Provides accessible defaults when no fallbacks are supplied:
  • loading: role="status" with “Loading image editor…”
  • error: role="alert" with a “Try again” button
• Exposes the original initialization error and a retry callback to functional error fallbacks.
• Keeps the editor mount element in the DOM beneath the fallback overlay, so initialization can continue without changing the editor’s container.
• Times out both reused and newly injected embed-script tags after 30 seconds. Previously, a CDN request that stalled without firing load or error left the component loading forever and made retry unreachable. The timed-out tag is removed so retry starts fresh.
• Documents the fallback API and adds tests for loading, error, retry, and stalled-script lifecycles.

Example

<ImageEditor
  image={url}
  loadingFallback={<Spinner label="Loading editor" />}
  errorFallback={(error, retry) => (
    <section role="alert">
      <p>Could not load the editor: {error.message}</p>
      <button onClick={retry}>Try again</button>
    </section>
  )}
/>

Backward compatibility

Existing ImageEditor usage is unchanged. Consumers who do not pass fallbacks receive sensible default loading and retryable error UI instead of a blank surface.

Verification

• npm test — 51 tests passing
• npm run typecheck
• npm run lint
• npm run build

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