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/src/i18n/locales/de/plex.json

@@ -27,6 +27,18 @@
   "watched": "Angesehen",
   "seasons": "{{count}} Staffeln",
   "playerSoon": "Player kommt bald — „{{title}}“",
+  "player": {
+    "loading": "Wird geladen…",
+    "back": "Zurück",
+    "skipIntro": "Intro überspringen",
+    "skipCredits": "Abspann überspringen",
+    "playPause": "Wiedergabe / Pause (Leertaste)",
+    "restart": "Von vorn beginnen",
+    "prev": "Vorherige Folge",
+    "next": "Nächste Folge",
+    "mute": "Stumm (m)",
+    "fullscreen": "Vollbild (f)"
+  },
   "playable": {
     "direct": "Spielt direkt im Browser",
     "remux": "Braucht ein leichtes Remux (keine Video-Neucodierung)",

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

@@ -27,6 +27,18 @@
   "watched": "Watched",
   "seasons": "{{count}} seasons",
   "playerSoon": "Player coming soon — “{{title}}”",
+  "player": {
+    "loading": "Loading…",
+    "back": "Back",
+    "skipIntro": "Skip intro",
+    "skipCredits": "Skip credits",
+    "playPause": "Play / pause (space)",
+    "restart": "Restart from beginning",
+    "prev": "Previous episode",
+    "next": "Next episode",
+    "mute": "Mute (m)",
+    "fullscreen": "Fullscreen (f)"
+  },
   "playable": {
     "direct": "Plays directly in the browser",
     "remux": "Needs a light remux (no video re-encode)",

frontend/src/i18n/locales/hu/plex.json

@@ -27,6 +27,18 @@
   "watched": "Megnézve",
   "seasons": "{{count}} évad",
   "playerSoon": "A lejátszó hamarosan jön — „{{title}}”",
+  "player": {
+    "loading": "Betöltés…",
+    "back": "Vissza",
+    "skipIntro": "Intro átugrása",
+    "skipCredits": "Stáblista átugrása",
+    "playPause": "Lejátszás / szünet (szóköz)",
+    "restart": "Újra az elejéről",
+    "prev": "Előző rész",
+    "next": "Következő rész",
+    "mute": "Némítás (m)",
+    "fullscreen": "Teljes képernyő (f)"
+  },
   "playable": {
     "direct": "Közvetlenül játszható a böngészőben",
     "remux": "Könnyű remux kell (nincs videó-újrakódolás)",