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/ProfileEditor.tsx

@@ -106,7 +106,12 @@ export default function ProfileEditor({ onClose }: { onClose: () => void }) {
   const patch = (p: Partial<DownloadSpec>) => setSpec((s) => ({ ...s, ...p }));
 
   return (
-    <Modal title={t("downloads.profiles.title")} onClose={onClose} maxWidth="max-w-xl">
+    <Modal
+      initialFocus="close"
+      title={t("downloads.profiles.title")}
+      onClose={onClose}
+      maxWidth="max-w-xl"
+    >
       {/* Existing formats */}
       <div className="space-y-1.5 mb-4">
         <SectionLabel level={3}>{t("downloads.profiles.builtin")}</SectionLabel>