refactor(ui): close the sort/layout reuse-gaps (R13 S2c step 5)

Two surfaces reinvented controls the app already consolidated:
- PlaylistsRail sort (a bespoke <select> + hand-rolled asc/desc flip) now uses
  the shared SortControl; "custom" is a directionless option (no arrow), the
  "unsynced first" pin rides the trailing slot as a pill toggle
- PlexPlaylistView Accordion/Tree layout (two hand-rolled toggle buttons) now
  uses the shared ViewSwitcher with a Rows3/ListTree option set

Adds aria-label i18n (plex.playlist.layoutLabel; HU+EN), drops the now-unused
ArrowUp/ArrowDown + ReactNode imports. Self-E2E'd the rail sort: key switch
reveals the arrow, asc<->desc flips, pin toggles, custom hides the arrow.
gate green (tsc/eslint-0/vitest 136/knip/prettier). Branch not merged.

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

@@ -218,6 +218,7 @@
     "playAll": "Play all",
     "delete": "Delete playlist",
     "empty": "This playlist is empty. Add titles from their info page.",
+    "layoutLabel": "Layout",
     "layoutAccordion": "Accordion view",
     "layoutTree": "Tree view",
     "removeShow": "Remove whole show",

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

@@ -218,6 +218,7 @@
     "playAll": "Összes lejátszása",
     "delete": "Lista törlése",
     "empty": "Ez a lista üres. Adj hozzá címeket az info-oldalukról.",
+    "layoutLabel": "Elrendezés",
     "layoutAccordion": "Harmonika nézet",
     "layoutTree": "Fa nézet",
     "removeShow": "Egész sorozat eltávolítása",