feat(plex): rework player timing (copyts) + player settings & personalization

Streaming / subtitle sync (the core fix):
- stream.py: add -copyts so HLS segments carry the true absolute PTS, and measure
  the real keyframe start K from seg_0 (ffprobe) -> return it as the session start.
  Fixes the seconds-long subtitle lead caused by using the requested seek offset
  instead of the keyframe ffmpeg actually lands on with video stream-copy.
- Add -noaccurate_seek so the re-encoded audio starts at the same keyframe as the
  video (was starting (X-K)s later -> seconds of silence after each seek/audio switch).
- Compensate the fixed ~1.0s lag hls.js introduces for non-zero-start copyts streams,
  folded into the session start so the clock, seeking and the subtitle shift are all
  content-accurate.
- /subtitle gains an offset param; _shift_vtt shifts absolute cues onto the session's
  zero-based clock and DROPS fully-past cue blocks (collapsing them to 0->0 made every
  past cue active at currentTime 0 on resume -> a pile-up until playback advanced).

Audio:
- /session + stream.py gain an audio A/V-sync offset (-itsoffset, full +/-, second
  input only when non-zero).

Player settings & personalization (per-account, persisted -> survive F5):
- storage.ts: useAccountPersistedObject (per-account JSON prefs blob).
- PlexPlayer: volume/mute, audio+subtitle language (index-based match, fixes the F5
  audio-revert), sync offsets, seek steps, subtitle style, auto-hide, play intent.
- Hotkeys A (cycle audio) / S (cycle subtitle), mouse-wheel volume, Ctrl+arrow fine
  seek, per-user plain/fine seek-step + auto-hide toggle.
- Subtitle appearance: size / colour / vertical position / background via ::cue + line.
- UI: split into a Tracks quick-menu + a tabbed gear panel (Sync | Playback | Subtitle);
  both dismiss on outside-click; edge-aware control tooltips.
- i18n en/hu/de for all new strings.

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

@@ -120,8 +120,25 @@
       "fullscreen": "Vollbild",
       "info": "Medieninfo",
       "back": "Stopp & zurück zum Feed",
-      "help": "Diese Hilfe anzeigen"
-    }
+      "help": "Diese Hilfe anzeigen",
+      "audio": "Audiospur wechseln",
+      "subtitle": "Untertitel wechseln",
+      "fineSeek": "Feines Springen"
+    },
+    "settings": "Einstellungen",
+    "sync": "Synchronität",
+    "subOffset": "Untertitel-Verzögerung",
+    "audioOffset": "Audio-Verzögerung",
+    "playback": "Wiedergabe",
+    "seekStep": "Sprungweite",
+    "fineSeek": "Feines Springen (Strg)",
+    "autoHide": "Steuerung ausblenden",
+    "subStyle": "Untertitel-Stil",
+    "subSize": "Schriftgröße",
+    "subPos": "Position",
+    "subColor": "Textfarbe",
+    "subBg": "Hintergrund",
+    "subBgNone": "Keiner"
   },
   "info": {
     "title": "Medieninfo",