🐛 Fixed design preview showing raw markdown when llms.txt is enabled (#28266)

fixes https://github.com/TryGhost/Ghost/issues/28265

- the design & branding and announcement bar previews fetch the page,
read it as text, and re-parse it as HTML into the iframe. They have
always sent `Accept: text/plain` purely as a "give me the raw body"
signal, which the frontend ignored
- the llms.txt feature added Accept-based content negotiation that now
treats `text/plain` as a request for the markdown variant, so with
labs.llmsTxt enabled these previews received markdown and rendered it as
garbled text
- the previews genuinely want HTML, so they now ask for `text/html`
instead of accidentally opting into markdown content negotiation