feat(plex): P2 subtitle + audio track selector (closes P2)

Backend: item detail returns audio_streams + subtitle_streams (from Plex Part.Stream
ordinals). The HLS session accepts audio/subtitle stream ordinals — the selected audio
is mapped (transcoded to AAC), a selected subtitle is muxed in as a WebVTT rendition
via a MASTER playlist (-master_pl_name + -var_stream_map sgroup:subs) so ffmpeg keeps
it in sync per-session. Selecting a track forces the HLS path (even for direct files).
Generic /stream/{rk}/hls/{filename} endpoint serves the master/media/vtt/ts artifacts.

Frontend: gear menu lists real audio + subtitle tracks; changing one restarts the
session at the current position (seek-restart mechanism). hls.js subtitle enabled on
SUBTITLE_TRACKS_UPDATED (not just MANIFEST_PARSED, else the VTT is never fetched).
Cues auto-nudged to line 88% (the full-height <video> clips a default bottom-edge cue).
Controls no longer auto-hide while the menu is open. plex.player.* i18n en/hu/de.

Verified in a real browser (2 Broke Girls S1E1, 2 audio + 2 subs): menu shows the real
tracks, enabling English subtitles renders them correctly positioned; backend master
playlist + vtt segments validated over HTTP.

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

@@ -43,7 +43,11 @@
     "next": "Next episode",
     "mute": "Mute (m)",
     "fullscreen": "Fullscreen (f)",
-    "download": "Download original file"
+    "download": "Download original file",
+    "tracks": "Audio & subtitles",
+    "audio": "Audio",
+    "subtitles": "Subtitles",
+    "subOff": "Off"
   },
   "playable": {
     "direct": "Plays directly in the browser",