Spinner: default assistive label bypasses the translation runtime

## Description

`Spinner` supplies the assistive-technology-facing default label `"Loading"` directly in component source instead of obtaining it from the Astryx translation runtime.

A localized application that relies on the default cannot translate the status. AT-facing text follows the same localization requirement as visible text. The default needs a catalog-backed translation path while preserving an explicit consumer `aria-label` override.

## Reproduction

1. Render `` inside an `InternationalizationProvider` using a non-English locale.
2. Inspect the Spinner accessible name.
3. Observe `"Loading"` in English.
4. Search `Spinner.tsx`; the default is a literal string rather than a translator key.

## Astryx Version

Current `main` (`@astryxdesign/core@0.5.4`).

## Environment

Chrome on macOS; DOM and source inspection.

Related: #3636