feat(playlists): queue playback in the player (auto-advance + prev/next)

PlayerModal now accepts an optional queue + startIndex. The active item drives the
player; it recreates per item (reusing the single-video resume / auto-watch / progress
logic), auto-advances to the next item when one ends, and shows Previous / Next controls
with an N / M indicator. The Playlists page passes the playlist as the queue from Play all
or a clicked row. Trilingual previous/next strings.

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

@@ -2,6 +2,8 @@
   "loading": "Loading…",
   "backToOriginal": "Back to the original video",
   "back": "Back",
+  "previous": "Previous",
+  "next": "Next",
   "close": "Close",
   "closeEsc": "Close (Esc)",
   "description": "Description",