Add copy and download actions for generated README output

[naheel0]

The generator currently shows the rendered and raw README output, but users need a quicker way to export the result into their own repository workflow.

Proposed scope

• Add a download button that saves the generated content as README.md.
• Keep the existing copy action available.
• Preserve the current preview/code toggle.

Acceptance criteria

• A user can download the generated markdown without leaving the page.
• The exported file is named README.md.
• The new action works for both preview and code output states.