ChatTypingIndicator: generated status sentences bypass the translation runtime

## Description

`ChatTypingIndicator` builds every assistive-technology-facing typing sentence directly in component source: `"is typing…"`, `"are typing…"`, `"Several people are typing…"`, and the English name-joining grammar.

A localized application cannot translate these generated status messages. AT-facing text follows the same localization requirement as visible text. The component needs catalog-backed messages and locale-aware list formatting.

## Reproduction

1. Render `ChatTypingIndicator` with one, two, and four names inside an `InternationalizationProvider` using a non-English locale.
2. Inspect the status text.
3. Observe the English sentences and English conjunction/comma grammar in every state.
4. Search `ChatTypingIndicator.tsx`; the phrases are literals and names are joined manually.

## Astryx Version

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

## Environment

Chrome on macOS; DOM and source inspection.

Related: #3636