feat(design): dialogs open by purpose and keep their commit row pinned (E28 S3f1)

Modal takes a required initialFocus (field / close / action), a field that
loads late takes the focus while it still rests on the close button, and
ModalFooter portals a dialog's commit row under the scrolling body from any
depth. The header and footer fades follow the body's scroll timeline, and the
viewport's interactive-widget=resizes-content keeps the footer above an
on-screen keyboard.

frontend/src/components/About.tsx

@@ -45,6 +45,7 @@ export default function About({
 
   return (
     <Modal
+      initialFocus="close"
       title={
         <span className="flex items-center gap-2">
           <Info className="w-5 h-5 text-accent" /> {t("about.title")}