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-lock.json

@@ -12,6 +12,7 @@
         "@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",
@@ -1855,6 +1856,12 @@
         "node": ">= 0.4"
       }
     },
+    "node_modules/hls.js": {
+      "version": "1.6.16",
+      "resolved": "https://registry.npmjs.org/hls.js/-/hls.js-1.6.16.tgz",
+      "integrity": "sha512-VSIRpLfRwlAAdGL4wiTucx2ScRipo0ed1FBatWkyt832jC4CReKstga6yIhYVwGu9LOBjuX9wzmRMeQdBJtzEA==",
+      "license": "Apache-2.0"
+    },
     "node_modules/html-parse-stringify": {
       "version": "3.0.1",
       "resolved": "https://registry.npmjs.org/html-parse-stringify/-/html-parse-stringify-3.0.1.tgz",