Restructure step 6: what's next and watch tracking

- What's next is the server's: state.advance counts down (10 s) when the
  media finishes with an Up next item first, holds while its performer or
  warm source is gone, and sets queuePlay at its end; cancel is shared, a
  seek or swap ends it; the server skips items nobody can put up. Shared
  rules in core whats_next.dart; the app's janitor, per-screen countdown
  and end-of-episode card are gone.
- The continuation (next Jellyfin episode, next file of a local show
  with its gaps) is an auto item under "Continues with", prepared like
  Up next's first item.
- Watch tracking by time In (D3): only time played while this viewer is
  in counts; away, paused, held, jumps and stalls never do (P7, P9).
  WatchTracker + device WatchHistory (files, links); Jellyfin progress
  reads the same watch; records are the AniList seam.
- A server restart no longer loses what played: apps hold their media
  and one offers it back (swap.restore, first offer wins), Up next is
  re-added, the source re-feeds, and the watch carries on.

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

docs/HOW-IT-WORKS.md

@@ -78,7 +78,13 @@ brings you back.
 6. **Swap.** Anyone can change the video by pasting another link. Everybody
    stops at 0 and the gate runs again for everyone there; ready flags stay
    as they were (a setting unreadies you instead).
-7. **Leave.** Closing the app or Leave says goodbye and drops you from the
+7. **What's next.** Up next is one list on the server for everyone, and
+   an episode you put up is followed by its next one. When a video ends
+   with something next, the server counts down 10 s on every screen;
+   anyone can cancel it for everyone (a seek stops it too). At zero the
+   app that has what plays next puts it up; if that person is away for a
+   moment, the countdown waits for them.
+8. **Leave.** Closing the app or Leave says goodbye and drops you from the
    server; the others keep going. If you vanish without a goodbye, after 15 s
    of silence you are "gone": you stop blocking but keep your place
    (ready flag, sitting out, Up next items, the video you pass on) for
@@ -90,6 +96,11 @@ anyone, and sit anyone out (the host too). Someone sitting out stays
 connected and in the chat, but their app loads nothing and nobody waits
 for them until they join back.
 
+Watched or not is counted by the time you were there while it played:
+sitting out, skipping around or a quick look never counts as watching.
+Jellyfin hears it for your account; files and links are remembered on
+your computer.
+
 ## Where video comes from
 
 - **Link**: you paste an http(s) URL. By default your computer opens it and