refactor(player): share the volume core across both players + extract PlexPlayerSettings (R12 S5 frontend)

Volume-consolidation (conservative — no behaviour or prefs-format change):
- playerVolume.ts is now scale-generic (a VolumeScale parameterises range /
  default / integer rounding). YT_SCALE (0-100 ints) and PLEX_SCALE (0..1
  float) bind the same tested core; the YouTube wrappers keep the original API
  so the existing tests are unchanged.
- The Plex usePlayerVolume hook drops its bespoke clamp01 arithmetic for the
  shared normalizeVolumeIn / stepVolumeIn at PLEX_SCALE. Side effect (a latent
  bug fix): a corrupt persisted volume now falls back to the default instead of
  a bare clamp silently muting it. The separate .muted flag stays Plex-only.
- clamp01 stays for the seek-bar fraction (its non-volume use).
- Tests: the generic core is covered at both scales (float no-round, corrupt ->
  default, step/clamp) — the Plex-side arithmetic finally has unit coverage.

PlexPlayerSettings extraction (verbatim JSX move, behaviour-equivalent):
- The ~240-line gear/tuning panel (sync / playback / subtitle / clock tabs)
  moves out of PlexPlayer into PlexPlayerSettings.tsx (a pure prefs+patchPrefs
  view owning no state). PlexPlayer shrinks 1402 -> 1173 lines; Slider/Toggle
  imports it no longer uses are dropped.

Gate green (tsc / eslint-0 / prettier / knip / vitest 83 / pytest 234). Dev
server smoke: app builds + loads clean, no console errors. The interactive
settings panel needs a playing Plex video (unreachable on localdev) -> owed to
the user's UAT.

frontend/src/lib/plexPlayerHooks.ts

@@ -17,7 +17,8 @@ import {
 } from "./fullscreen";
 import { api, type PlexItemDetail, type PlexMarker } from "./api";
 import { qk } from "./queryKeys";
-import { clamp01, type PlexPlayerPrefs } from "./plexPlayerPrefs";
+import { type PlexPlayerPrefs } from "./plexPlayerPrefs";
+import { PLEX_SCALE, normalizeVolumeIn, stepVolumeIn } from "./playerVolume";
 
 // A live wall clock for the top bar, re-rendered every 15s (minute resolution is enough for the
 // HH:MM display, and a coarse tick keeps the whole player from re-rendering each second).
@@ -138,9 +139,10 @@ export function usePlexProgress(opts: {
 
 // Volume/mute for the Plex player's HTML5 <video> (0..1 float, independent .muted flag). Persists to
 // the per-account prefs (survives F5) and re-applies on each fresh session (the element resets to its
-// default volume after a load/seek-restart). NOTE: this is the Plex-side model; the YouTube player has
-// its own 0-100 / no-mute-flag arithmetic in playerVolume.ts. Sharing the tested pure core across both
-// is a separate R12 consolidation sprint — this hook is the seam it will rework internally.
+// default volume after a load/seek-restart). The 0..1 arithmetic is the shared, unit-tested core in
+// playerVolume.ts bound to PLEX_SCALE (the YouTube player binds the same core to YT_SCALE, 0–100); the
+// separate .muted flag stays here since it's Plex-only. Using normalizeVolumeIn (not a bare clamp)
+// also means a corrupt persisted volume falls back to the default instead of silently muting.
 export function usePlayerVolume(opts: {
   videoRef: RefObject<HTMLVideoElement | null>;
   volume: number;
@@ -160,7 +162,7 @@ export function usePlayerVolume(opts: {
     (vol: number, mute: boolean) => {
       const v = videoRef.current;
       if (!v) return;
-      v.volume = clamp01(vol);
+      v.volume = normalizeVolumeIn(vol, undefined, PLEX_SCALE);
       v.muted = mute;
     },
     [videoRef],
@@ -169,12 +171,15 @@ export function usePlayerVolume(opts: {
   const setVol = useCallback(
     (vol: number, mute: boolean) => {
       applyVolume(vol, mute);
-      patchPrefs({ volume: clamp01(vol), muted: mute });
+      patchPrefs({ volume: normalizeVolumeIn(vol, undefined, PLEX_SCALE), muted: mute });
     },
     [applyVolume, patchPrefs],
   );
   const nudgeVolume = useCallback(
-    (d: number) => setVol(clamp01(volume + d), clamp01(volume + d) === 0),
+    (d: number) => {
+      const next = stepVolumeIn(volume, { delta: d, fallback: volume }, PLEX_SCALE);
+      setVol(next, next === 0);
+    },
     [volume, setVol],
   );
   const toggleMute = useCallback(() => setVol(volume, !muted), [volume, muted, setVol]);