fix: add OK button back to audiobook sleep timer custom input

Mobile needs the button since there's no keyboard Enter key in all cases.
Button is compact and styled to match the accent color.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -722,6 +722,7 @@
         <div class="abs-sleep-custom-row">
           <span class="abs-sleep-custom-label">Eigene Zeit</span>
           <input id="abs-sleep-custom-input" type="number" min="1" max="999" placeholder="Min" onkeydown="if(event.key==='Enter'){var v=parseInt(this.value);if(v>0)absSleepIn(v);}">
+          <button class="abs-sleep-custom-ok" onclick="var v=parseInt(document.getElementById('abs-sleep-custom-input').value);if(v>0)absSleepIn(v);">OK</button>
         </div>
       </div>
     </div>