Chapters on the scrubber, Skip OP/ED, audio/subtitle menus

- PlaybackController exposes chapters, segments (Jellyfin
  MediaSegments for account holders), tracks, position/duration.
- Own seek bar with chapter diamonds and chapter title on hover,
  seeking through the sync user-seek path.
- Skip OP / Skip ED pills from Intro/Outro segments or chapter names
  (ED skips to its end only); Skip 1:30 fallback for >20 min videos
  without OP info; next-episode pill now starts at the ED.
- PgUp/PgDn chapters, Shift+Right +1:30; Audio and Subtitles menus
  with count badges via the existing track path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ROADMAP.md

@@ -49,8 +49,6 @@ in a session"):
 - Queue: put several library items (a season, a pick list) up as a
   playlist that advances like a swap (shares `queue-add` / `queue-skip`
   with the backlog's Playlist item).
-- The "Next episode" prompt from the item's Outro / credits segment instead
-  of the last two minutes (`nextPromptStart`).
 - Auto-advance cancel for everyone: today Cancel is per screen (the
   designated swapper's Cancel stops the swap; a seek stops all). A shared
   cancel needs a wire message.