Proposal editor uses the Documents editor

Internal: the proposal editor (site-admin preview) uses the Documents editor's chrome and code: the File, Edit, Insert and Format menus, the shared formatting toolbar (partials/editor/text-toolbar.twig) and Link drawer (partials/editor/link-drawer.twig), and Documents' text editing, now in public/assets/document-text.js, which document-editor.js also uses. The File menu holds what the More menu held. Each section has a bar at its top with its grip, its kind, Comment, its arrows and a ⋯ menu, and its title reads as a heading. Pricing lines are a two-row line-item table, packages are not boxed, and short answers are rows. Fixed: the pricing table's column headings ran together, and a textarea in a line-item table had no vertical padding. AGENTS.md and docs/DESIGN_PRINCIPLES.md say that writing uses one of two editors, Documents or Notes, with their code.