docs: publish the UI standards in docs/contributing

The binding UI rulebook (card anatomy, spacing scale, text colour,
tokens, primitive mapping, enforcement) lived outside the repository,
so a contributor could follow a code comment to a file they could not
open. It now lives at docs/contributing/ui-standards.md, every comment
that cited it points there, and the docs index and ui-guidelines.md
say which document wins when they disagree.

src/components/module-list/module-order-editor.tsx

@@ -110,7 +110,7 @@ export function ModuleOrderEditor({ items, onChange }: ModuleOrderEditorProps) {
     // `relative` anchors the sr-only drag-hint paragraph: `sr-only` is
     // absolutely positioned, and without a positioned ancestor its 1px box
     // lands below the list in DOCUMENT coordinates and makes the document
-    // itself scrollable (second scrollbar — UI-STANDARDS §9 one-scroll-floor).
+    // itself scrollable (second scrollbar — ui-standards.md §9 one-scroll-floor).
     <div className="relative space-y-2" data-slot="module-order-editor">
       <DndContext
         sensors={sensors}