Jellyfin end to end: per-viewer playback, progress, episodes, swap browser

- Host/swap send refs only; pasted Jellyfin links become refs; the host
  hands the relay its own app-obtained credential.
- Each viewer resolves with their own account (token in a header) or
  via the relay, else a clear blocked card with Link account.
- Per-account progress reporting (Playing/Progress/Stopped), 401 marks
  the account expired.
- Previous/next episode in the controls (< > / Shift+P/N), floating
  Next episode pill near the end, auto-advance countdown (one
  designated swapper), track carry-over per series.
- Swap panel uses the shared MediaPicker; browser location remembered
  per server and opens on the current series.
- Fix: states are applied one at a time (a stale pause could land
  after a newer play).

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

docs/DEVELOPMENT.md

@@ -46,7 +46,8 @@ and `e2e_check` (simulated viewers against a live server).
 - `cd packages/core && dart test`, `cd packages/server && dart test`.
 - `cd apps/desktop && flutter test test/<file>`.
 - Real-mpv tests: `sync_playback_e2e`, `coord_presence`, `video_reload_e2e`,
-  `seek_drag_e2e`, `end_of_file_e2e`, `readahead_e2e`. The sync and
+  `seek_drag_e2e`, `end_of_file_e2e`, `readahead_e2e`, `jellyfin_e2e` (a
+  local fake Jellyfin over HTTP; tests never talk to a real Jellyfin). The sync and
   end-of-file ones expect `~/Videos/chiyo.mp4`; others generate media with
   ffmpeg into `WT_TEST_TMP`. Headless mpv (no video output attached) won't
   open video-only files, so test media needs an audio track. Tests always log