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/playerVolume.ts

@@ -1,33 +1,68 @@
-// The YouTube player's volume arithmetic, kept out of PlayerModal so it can be unit-tested. Every
+// Volume arithmetic for BOTH players, kept out of the components so it can be unit-tested. Every
 // rule here was established by a bug: a wheel spin that lost notches to the iframe's lagging
 // getVolume, a partial stored object that became NaN and persisted as `null`, and a "corrupt →
 // default" fallback that jumped an already-quiet session to full blast.
+//
+// The two players run on different native scales — the YouTube iframe API takes 0–100 integers, the
+// HTML5 <video> element a 0..1 float with a separate `.muted` flag — so the pure core below is
+// scale-agnostic (a `VolumeScale` parameterises the range, default, and integer rounding) and each
+// player binds it to its own scale. The YouTube-bound wrappers keep the original 0–100 API.
 
-/** Level used when nothing trustworthy is available. */
-export const DEFAULT_VOLUME = 100;
+/** A player's volume scale. `round` is true for the integer YouTube API, false for the 0..1 float. */
+export interface VolumeScale {
+  min: number;
+  max: number;
+  /** Level used when nothing trustworthy is available. */
+  default: number;
+  round: boolean;
+}
 
-const clamp = (n: number) => Math.max(0, Math.min(100, Math.round(n)));
+/** YouTube iframe API: 0–100 integers, no mute flag (level 0 IS muted). */
+export const YT_SCALE: VolumeScale = { min: 0, max: 100, default: 100, round: true };
+/** HTML5 <video>: a 0..1 float (mute is a separate flag the caller carries). */
+export const PLEX_SCALE: VolumeScale = { min: 0, max: 1, default: 1, round: false };
+
+function clampTo(n: number, s: VolumeScale): number {
+  const c = Math.max(s.min, Math.min(s.max, n));
+  return s.round ? Math.round(c) : c;
+}
 
 /**
- * Coerce anything that claims to be a volume into a usable 0–100 level.
+ * Coerce anything that claims to be a volume into a usable level ON `scale`.
  *
  * Only a real, finite `number` is accepted: the stored blob is JSON, so a level is a number or it
  * is corrupt — `null`, `undefined`, `"80"` and `NaN` all mean "we don't know", and `Number(null)`
  * being `0` (silently muting the player) is exactly the coercion to avoid. `fallback` is the last
  * level known to work; it is itself validated, so a corrupt value can never travel through it.
  */
-export function normalizeVolume(raw: unknown, fallback: unknown = DEFAULT_VOLUME): number {
-  if (typeof raw === "number" && Number.isFinite(raw)) return clamp(raw);
-  if (typeof fallback === "number" && Number.isFinite(fallback)) return clamp(fallback);
-  return DEFAULT_VOLUME;
+export function normalizeVolumeIn(raw: unknown, fallback: unknown, scale: VolumeScale): number {
+  if (typeof raw === "number" && Number.isFinite(raw)) return clampTo(raw, scale);
+  if (typeof fallback === "number" && Number.isFinite(fallback)) return clampTo(fallback, scale);
+  return scale.default;
 }
 
-/** One volume step (wheel notch / arrow key) from `current`.
+/** One volume step (wheel notch / arrow key) from `current`, on `scale`.
  *
  * `delta` and `fallback` ride in an options object on purpose: as two adjacent `number`
  * parameters they were transposable without a type error, and swapping them silently steps the
  * volume BY the current level. */
+export function stepVolumeIn(
+  current: unknown,
+  opts: { delta: number; fallback: unknown },
+  scale: VolumeScale,
+): number {
+  return clampTo(normalizeVolumeIn(current, opts.fallback, scale) + opts.delta, scale);
+}
+
+// --- YouTube-bound convenience wrappers (0–100 integers) — the original API, behaviour unchanged. ---
+
+/** Level used when nothing trustworthy is available (YouTube scale). */
+export const DEFAULT_VOLUME = YT_SCALE.default;
+
+export function normalizeVolume(raw: unknown, fallback: unknown = DEFAULT_VOLUME): number {
+  return normalizeVolumeIn(raw, fallback, YT_SCALE);
+}
+
 export function stepVolume(current: unknown, opts: { delta: number; fallback: unknown }): number {
-  const base = normalizeVolume(current, opts.fallback);
-  return clamp(base + opts.delta);
+  return stepVolumeIn(current, opts, YT_SCALE);
 }