feat: add custom minutes input to audiobook sleep timer

index.html

@@ -696,6 +696,10 @@
         <button onclick="absSleepIn(30)">30 Min</button>
         <button onclick="absSleepIn(45)">45 Min</button>
         <button onclick="absSleepIn(60)">60 Min</button>
+        <div class="abs-sleep-custom">
+          <input id="abs-sleep-custom-input" type="number" min="1" max="999" placeholder="Min" onkeydown="if(event.key==='Enter'){const v=parseInt(this.value);if(v>0)absSleepIn(v);}">
+          <button onclick="const v=parseInt(document.getElementById('abs-sleep-custom-input').value);if(v>0)absSleepIn(v);">OK</button>
+        </div>
       </div>
     </div>