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/lib/api.ts

@@ -1218,14 +1218,17 @@ export const api = {
     req(`/api/plex/show/${encodeURIComponent(id)}`),
   plexItem: (id: string): Promise<PlexItemDetail> =>
     req(`/api/plex/item/${encodeURIComponent(id)}`),
-  plexSession: (id: string, start = 0, audio?: number | null): Promise<PlexPlaySession> => {
+  plexSession: (id: string, start = 0, audio?: number | null, aoff = 0): Promise<PlexPlaySession> => {
     const p = new URLSearchParams({ start: String(Math.max(0, Math.floor(start))) });
     if (audio != null) p.set("audio", String(audio));
+    if (aoff) p.set("aoff", String(aoff));
     return req(`/api/plex/stream/${encodeURIComponent(id)}/session?${p.toString()}`, { method: "POST" });
   },
   // WebVTT URL for a text subtitle track (used directly as a <track src>). Same-origin → cookie-authed.
-  plexSubtitleUrl: (id: string, ord: number): string =>
-    `/api/plex/subtitle/${encodeURIComponent(id)}/${ord}`,
+  // `offset` = the session's real media start (0 for direct-play); the backend shifts the absolute
+  // cue times onto the HLS session's zero-based clock so subtitles line up with speech after a seek.
+  plexSubtitleUrl: (id: string, ord: number, offset = 0): string =>
+    `/api/plex/subtitle/${encodeURIComponent(id)}/${ord}${offset ? `?offset=${offset}` : ""}`,
   plexProgress: (id: string, position_seconds: number, duration_seconds: number): Promise<unknown> =>
     req(`/api/plex/item/${encodeURIComponent(id)}/progress`, {
       method: "POST",

frontend/src/lib/storage.ts

@@ -1,4 +1,4 @@
-import { useState } from "react";
+import { useCallback, useState } from "react";
 
 // Central registry of every localStorage key the app uses. One place to see them all, rename
 // safely, and avoid collisions — instead of `"siftlode.x"` string literals scattered across
@@ -158,3 +158,24 @@ export function useAccountPersistedState(
   };
   return [value, set];
 }
+
+/** A per-account persisted JSON OBJECT: `patch(partial)` merges + saves, unknown/missing keys fall
+ * back to `defaults` (via readAccountMerged) so adding a field later is safe. Used for bundles of
+ * related per-account settings (e.g. the Plex player prefs: volume, offsets, seek steps, sub style). */
+export function useAccountPersistedObject<T extends object>(
+  base: string,
+  defaults: T,
+): [T, (patch: Partial<T>) => void] {
+  const [value, setValue] = useState<T>(() => readAccountMerged(base, defaults));
+  const patch = useCallback(
+    (p: Partial<T>) => {
+      setValue((prev) => {
+        const next = { ...prev, ...p };
+        writeAccount(base, next);
+        return next;
+      });
+    },
+    [base],
+  );
+  return [value, patch];
+}