React Native: Implement Markdown

Background

Today, HTML version of Web Chat has this function. It returns a HTML string.

function renderMarkdown(markdown: string): string;

We might need/want to change this signature so it returns a React element.

function renderMarkdown(markdown: string, onNavigate: EventHandler): React.Element;

(The signature is not finalized. In HTML, we will render <a target="_blank"> and let the browser handle it. In React Native, we might render something else, and handle window.open differently than in HTML.)

When some links are clicked, we will need to customize the navigation experience:

• window.open can be overridden
• Some links need to handle programmatically, e.g. Send failed, [#retry](retry)

The issue does not name files or tests; start by locating the existing HTML renderMarkdown implementation and the React Native rendering path. Clarify the final return signature and navigation behavior, including window.open overrides and programmatic links such as retry, then define completion around rendered Markdown and link handling.