feat(playlists): YouTube-mirrored playlists in the UI (read-only) + sync button

Show YouTube-sourced playlists with a YouTube icon; they're read-only for now (no
rename/delete/reorder/remove, excluded from the add-to-playlist popover) with a
'synced from YouTube' note — editing comes with the write-back phase. Add a 'Sync from
YouTube' button in the Playlists rail (api.syncYoutubePlaylists) with a result toast.
Trilingual strings.

frontend/src/i18n/locales/de/playlists.json

@@ -1,6 +1,10 @@
 {
   "title": "Wiedergabelisten",
   "watchLater": "Später ansehen",
+  "syncYoutube": "Von YouTube synchronisieren",
+  "syncedToast": "{{count}} Wiedergabelisten von YouTube synchronisiert",
+  "syncFailed": "Synchronisierung von YouTube fehlgeschlagen",
+  "ytReadonly": "Von YouTube synchronisiert — vorerst schreibgeschützt",
   "noneYet": "Noch keine Wiedergabelisten",
   "newPlaylist": "Neue Liste…",
   "itemCount_one": "{{count}} Video",

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

@@ -1,6 +1,10 @@
 {
   "title": "Playlists",
   "watchLater": "Watch later",
+  "syncYoutube": "Sync from YouTube",
+  "syncedToast": "Synced {{count}} playlists from YouTube",
+  "syncFailed": "Couldn't sync from YouTube",
+  "ytReadonly": "Synced from YouTube — read-only for now",
   "noneYet": "No playlists yet",
   "newPlaylist": "New playlist…",
   "itemCount_one": "{{count}} video",

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

@@ -1,6 +1,10 @@
 {
   "title": "Lejátszási listák",
   "watchLater": "Megnézem később",
+  "syncYoutube": "Szinkron YouTube-ról",
+  "syncedToast": "{{count}} lista szinkronizálva a YouTube-ról",
+  "syncFailed": "Nem sikerült a YouTube-szinkron",
+  "ytReadonly": "YouTube-ról szinkronizálva — egyelőre csak olvasható",
   "noneYet": "Még nincs lejátszási lista",
   "newPlaylist": "Új lista…",
   "itemCount_one": "{{count}} videó",