feat(player): sleep timer and delayed start via long-press on play/pause (#270)

Add scheduled pause and resume timers in the player store, cleared on stop
and track changes. Long-press opens a compact preset modal anchored above
the transport row; one-tap presets plus custom minutes. Portaled countdown
badge on the play button; clear the long-press click guard when the modal
closes so the first play/pause click works after scheduling.

src/utils/playbackScheduleFormat.ts

@@ -0,0 +1,13 @@
+/** Remaining time until wall-clock `deadlineMs` (m:ss or h:mm:ss). */
+export function formatPlaybackScheduleRemaining(deadlineMs: number | null, nowMs: number): string {
+  if (deadlineMs == null) return '';
+  const sec = Math.max(0, Math.ceil((deadlineMs - nowMs) / 1000));
+  const m = Math.floor(sec / 60);
+  const s = sec % 60;
+  if (m >= 60) {
+    const h = Math.floor(m / 60);
+    const rm = m % 60;
+    return `${h}:${rm.toString().padStart(2, '0')}:${s.toString().padStart(2, '0')}`;
+  }
+  return `${m}:${s.toString().padStart(2, '0')}`;
+}