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/i18n/locales/de/plex.json

@@ -158,7 +158,8 @@
     "newPlaceholder": "Name der neuen Playlist…",
     "create": "Erstellen",
     "none": "Noch keine Playlists. Erstelle oben eine.",
-    "count": "{{count}} Titel"
+    "count": "{{count}} Titel",
+    "groupProgress": "{{in}} / {{size}}"
   },
   "playlist": {
     "section": "Playlists",
@@ -172,6 +173,16 @@
     "empty": "Diese Playlist ist leer. Füge Titel über deren Infoseite hinzu.",
     "up": "Nach oben",
     "down": "Nach unten",
-    "remove": "Entfernen"
+    "remove": "Entfernen",
+    "layoutAccordion": "Akkordeon-Ansicht",
+    "layoutTree": "Baum-Ansicht",
+    "removeShow": "Ganze Serie entfernen",
+    "removeSeason": "Ganze Staffel entfernen",
+    "episodes": "{{count}} Folgen",
+    "season": "Staffel {{n}}",
+    "seasonUnknown": "Folgen",
+    "addShow": "Ganze Serie zu einer Playlist",
+    "addSeason": "Staffel zu einer Playlist",
+    "addEpisode": "Folge zu einer Playlist"
   }
 }