Reels: timed recording with auto-stop

recStart now carries the format and an optional maxMs; the output stops
itself when the time is up (15/30/60/90s picker in the control UI), so a
reel doesn't depend on a manual stop.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/control.html

@@ -320,6 +320,14 @@
           <option value="1080x1440">3:4 verticale (1080×1440)</option>
           <option value="2560x1080">21:9 cinema (2560×1080)</option>
         </select>
+        <label>Durata (Reel: si ferma da solo)</label>
+        <select id="rec-dur">
+          <option value="0">∞ manuale</option>
+          <option value="15000">⏱ 15 secondi</option>
+          <option value="30000">⏱ 30 secondi</option>
+          <option value="60000">⏱ 60 secondi</option>
+          <option value="90000">⏱ 90 secondi</option>
+        </select>
         <button id="btn-rec" class="wide">🔴 Avvia registrazione</button>
         <button id="btn-rec-folder" class="wide">📁 Apri cartella registrazioni</button>
         <div class="hint-mini">Registra ciò che vedi sull'output + l'audio (file o input live). Salvataggio in <b>Filmati ▸ DJ Visualizer</b>. La conversione in MP4 avviene allo stop.</div>