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/LibraryPlayer.tsx

@@ -41,7 +41,7 @@ export default function LibraryPlayer({ job, onClose }: { job: DownloadJob; onCl
   );
 
   return (
-    <Modal title={jobTitle(job)} onClose={onClose} maxWidth="max-w-4xl">
+    <Modal initialFocus="close" title={jobTitle(job)} onClose={onClose} maxWidth="max-w-4xl">
       {isAudio ? (
         <div className="flex flex-col items-center gap-4 py-2">
           {poster ? (