Structured content editor

The shared Tiptap/Markdown editor owns safe document conversion, unsaved source buffers and localized controls. Products and app rich-text fields use this same editor and the existing Rust rich-content validator; neither accepts executable HTML.

Visual editing, Markdown and Preview are directly available in the shared mode bar. Preview uses RichDescription, leaves the draft unchanged and hides mutation controls until returning to the editor. Pending Markdown buffers block preview so unapplied source is not discarded.