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/PlexCollectionEditor.tsx

@@ -144,7 +144,11 @@ export default function PlexCollectionEditor({
   }
 
   return (
-    <Modal title={t("plex.collEditor.title", { title: item.title })} onClose={onClose}>
+    <Modal
+      initialFocus="field"
+      title={t("plex.collEditor.title", { title: item.title })}
+      onClose={onClose}
+    >
       {/* Create new */}
       <div className="mb-3 flex gap-2">
         <Input