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/ShareDialog.tsx

@@ -349,7 +349,7 @@ function LinkShare({ job }: { job: DownloadJob }) {
 export default function ShareDialog({ job, onClose }: { job: DownloadJob; onClose: () => void }) {
   const { t } = useTranslation();
   return (
-    <Modal title={t("downloads.share.title")} onClose={onClose}>
+    <Modal initialFocus="close" title={t("downloads.share.title")} onClose={onClose}>
       <div className="space-y-5">
         <UserShare job={job} />
         <div className="border-t border-border" />