Add a rendered markdown view toggle in the preview

[naheel0]

The preview is useful, but some users want to inspect the rendered README separately from the raw markdown source.

Proposed scope

• Add a toggle between rendered and raw markdown views.
• Make the active mode obvious in the preview header.
• Preserve the current copy/download controls.

Acceptance criteria

• Users can switch between rendered and raw output without regenerating content.
• The toggle state updates the existing preview panel only.
• Existing markdown sanitization continues to protect the rendered view.