Rich text composer on iOS with HTML round-tripping

Replying to an HTML email on iOS used to dump raw <br> and <div style='...'> tags into a plain TextField. Not great.

The new RichTextEditor is a UIViewRepresentable wrapping UITextView with two-way HTML round-tripping:

• On first layout it parses the server HTML via NSAttributedString(data:options:) with DocumentType.html, then normalises the typography to the system body font + UIColor.label so the quoted block doesn't render as Times New Roman 12pt on dark mode.
• allowsEditingTextAttributes = true surfaces the iOS selection menu with Bold / Italic / Underline for free — no custom toolbar needed for MVP.
• On change, textViewDidChange serialises back to HTML via attributedText.data(from:documentAttributes:) and writes to the SwiftUI binding on the next runloop tick (avoids feedback loops with updateUIView).

The backend /mail/send endpoint already treats the body as HTML, so outgoing mail looks the same whether you composed it on the web or on iPhone.