feat(plex): make the playlists rail widget a compact combobox

The playlists section used to render an always-open list plus a dedicated
create input, growing the rail taller with every playlist. Replace it with a
single Combobox field (reusing the shared component, like the feed's "Your
tags"): the list of playlists only appears when the dropdown is opened, and
typing a name that doesn't exist offers a "Create …" row. Picking a playlist
opens it (the existing playlistOpen hand-off); the create row creates then opens
the new one. `selected` stays empty — this picks an action, not a persisted value.

frontend/src/i18n/locales/en/plex.json

@@ -243,6 +243,8 @@
   "playlist": {
     "section": "Playlists",
     "newPlaceholder": "New playlist…",
+    "pickerPlaceholder": "Find or create a playlist…",
+    "createNamed": "Create \"{{name}}\"",
     "create": "Create playlist",
     "none": "No playlists yet.",
     "deleteConfirm": "Delete the playlist \"{{title}}\"?",

frontend/src/i18n/locales/hu/plex.json

@@ -243,6 +243,8 @@
   "playlist": {
     "section": "Lejátszási listák",
     "newPlaceholder": "Új lista…",
+    "pickerPlaceholder": "Lista keresése vagy létrehozása…",
+    "createNamed": "„{{name}}” létrehozása",
     "create": "Lista létrehozása",
     "none": "Még nincs lista.",
     "deleteConfirm": "Törlöd a(z) „{{title}}” listát?",