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/en/plex.json

@@ -158,7 +158,8 @@
     "newPlaceholder": "New playlist name…",
     "create": "Create",
     "none": "No playlists yet. Create one above.",
-    "count": "{{count}} items"
+    "count": "{{count}} items",
+    "groupProgress": "{{in}} / {{size}}"
   },
   "playlist": {
     "section": "Playlists",
@@ -172,6 +173,16 @@
     "empty": "This playlist is empty. Add titles from their info page.",
     "up": "Move up",
     "down": "Move down",
-    "remove": "Remove"
+    "remove": "Remove",
+    "layoutAccordion": "Accordion view",
+    "layoutTree": "Tree view",
+    "removeShow": "Remove whole show",
+    "removeSeason": "Remove whole season",
+    "episodes": "{{count}} episodes",
+    "season": "Season {{n}}",
+    "seasonUnknown": "Episodes",
+    "addShow": "Add whole show to a playlist",
+    "addSeason": "Add season to a playlist",
+    "addEpisode": "Add episode to a playlist"
   }
 }