Mobile: timed per-track scenes + visual interludes on the remote

Port the Mac app's scene system to the phone control. Each queue entry is
now an independent clone carrying a `cues` list of timed elements (effect /
text / image), each with its own appearance time and visible duration; the
three channels are driven independently from playback progress (advanceCues).
Add visual-only "Intermezzo" queue items (silence audio, run their scene for
a set duration) and make the inter-track gap keep the scene running. New
mobile scene editor lives in the queue card detail (⚙). The queue (with
scenes) is persisted via saveState so it survives a phone reload.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/index.html

@@ -48,7 +48,9 @@
           <button id="np-next">⏭</button>
         </div>
       </div>
-      <label class="lbl">Coda <span id="queue-count">0</span></label>
+      <label class="lbl">Coda <span id="queue-count">0</span>
+        <button id="add-interlude" class="mini" title="Aggiungi un intermezzo visivo (solo scena, niente audio)">➕ Intermezzo</button>
+      </label>
       <div id="queue" class="list"></div>
       <label class="lbl">Libreria sul dispositivo</label>
       <div id="lib" class="list"></div>