refactor(player): E.9 — extract scheduled pause/resume timer lifecycle (#572)

Two timer mutables (`scheduledPauseTimer`, `scheduledResumeTimer`) plus
the three clear helpers move into `src/store/scheduleTimers.ts`. The
module also gains `schedulePauseTimer(delayMs, onFire)` /
`scheduleResumeTimer(delayMs, onFire)` so callers no longer need to do
the `window.setTimeout(...) as unknown as number` cast or null the
handle inside the fire callback — the module auto-clears its own
reference before invoking the user callback.

`schedulePauseIn` / `scheduleResumeIn` store actions are now four lines
shorter each and don't reach into the timer mutables.

playerStore 3389 → 3369 LOC. 9 focused tests cover schedule + fire +
clear + replace-on-reschedule + independence between the two timers.

src/store/scheduleTimers.ts

@@ -0,0 +1,50 @@
+/**
+ * Deferred pause / resume timers — back the `schedulePauseIn` /
+ * `scheduleResumeIn` store actions. Encapsulated so the timer handles
+ * never leak: every public API either schedules + auto-clears on fire,
+ * or clears an outstanding timer outright. Cleared on stop, new track,
+ * manual pause/resume.
+ */
+
+let scheduledPauseTimer: number | null = null;
+let scheduledResumeTimer: number | null = null;
+
+export function schedulePauseTimer(delayMs: number, onFire: () => void): void {
+  clearScheduledPauseTimers();
+  scheduledPauseTimer = window.setTimeout(() => {
+    scheduledPauseTimer = null;
+    onFire();
+  }, delayMs) as unknown as number;
+}
+
+export function scheduleResumeTimer(delayMs: number, onFire: () => void): void {
+  clearScheduledResumeTimers();
+  scheduledResumeTimer = window.setTimeout(() => {
+    scheduledResumeTimer = null;
+    onFire();
+  }, delayMs) as unknown as number;
+}
+
+export function clearScheduledPauseTimers(): void {
+  if (scheduledPauseTimer != null) {
+    window.clearTimeout(scheduledPauseTimer);
+    scheduledPauseTimer = null;
+  }
+}
+
+export function clearScheduledResumeTimers(): void {
+  if (scheduledResumeTimer != null) {
+    window.clearTimeout(scheduledResumeTimer);
+    scheduledResumeTimer = null;
+  }
+}
+
+export function clearAllPlaybackScheduleTimers(): void {
+  clearScheduledPauseTimers();
+  clearScheduledResumeTimers();
+}
+
+/** Test-only: clear both handles without invoking the timer callbacks. */
+export function _resetScheduleTimersForTest(): void {
+  clearAllPlaybackScheduleTimers();
+}