feat(plex): P2 player — rich full-page HLS/direct player + watch-state

- backend: GET /api/plex/item/{rk} detail (metadata, cast + intro/credit markers
  from Plex, episode prev/next, per-user resume+status); POST /item/{rk}/progress
  (resume checkpoint, near-end→watched) + /state (new|watched|hidden).
- frontend PlexPlayer.tsx (lazy, pulls hls.js): full-page player over the Plex
  module. Direct files stream raw <video>; remux via hls.js on the seek-restart
  session — custom controls map the ABSOLUTE timeline over the session offset, and a
  seek beyond the loaded region restarts the session at the target. Play/pause/resume
  /restart, ±10s seek, prev/next episode (Shift+←/→), volume, windowed/fullscreen,
  full keyboard, Skip intro/Skip credits from markers (shaded on the seekbar),
  auto-advance + watched-on-end, resume from saved position, 10s progress checkpoints.
- PlexBrowse: card/episode click opens the player as a history subview (browser Back
  returns to the grid/show). api client (plexItem/plexSession/plexProgress/plexSetState)
  + types; plex.player.* i18n en/hu/de. hls.js@^1.6.16.
- Verified over HTTP: detail w/ markers+cast+episode-nav, progress persist. Video
  playback itself is pending browser UAT. Subtitle/audio track SELECTION deferred.

frontend/package.json

@@ -14,6 +14,7 @@
     "@tanstack/react-query": "^5.51.0",
     "@tanstack/react-virtual": "^3.14.3",
     "clsx": "^2.1.1",
+    "hls.js": "^1.6.16",
     "i18next": "^23.11.5",
     "lucide-react": "^0.408.0",
     "react": "^18.3.1",