feat(plex): grouped collapsible playlist view + drag-and-drop + bulk add

Playlist view now groups a run of episodes from the same show into a
collapsible season/show block so a whole series doesn't sprawl into an
endless flat list; movies stay standalone (larger poster card in the
accordion). Two per-account layouts — Accordion and Tree — persisted via
LS.plexPlaylistLayout; show groups start collapsed. Reorder is drag &
drop (@dnd-kit): a show block moves as one unit, episodes reorder within
their show, keyboard-draggable via KeyboardSensor. Remove works per item,
per season, or per whole show. The add-to-playlist dialog is generalised
to a single leaf or a whole group (tri-state none/some/all with an in/size
count); the show page gains add buttons for an episode, a season, and the
whole show. i18n en/hu/de.

frontend/src/components/PlexInfo.tsx

@@ -526,7 +526,10 @@ export default function PlexInfo({
         />
       )}
       {playlistOpen && (
-        <PlexPlaylistAdd item={{ id: detail.id, title: detail.title }} onClose={() => setPlaylistOpen(false)} />
+        <PlexPlaylistAdd
+          target={{ kind: "single", ratingKey: detail.id, title: detail.title }}
+          onClose={() => setPlaylistOpen(false)}
+        />
       )}
     </div>
   );