Per-track scenes: auto-apply effect + text + image when a playlist track starts

Each playlist track gets an optional scene (🎬) bound to it: pick the current
effect, a ticker text and an overlay image. When the track plays, the scene is
applied automatically — a cue/queue per song. Scene image is a dedicated output
layer (also captured in recordings).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/control.html

@@ -52,8 +52,9 @@
           <label class="chk"><input type="checkbox" id="repeat-playlist" /> Ripeti</label>
         </div>
         <input type="file" id="tracks-input" accept="audio/*" multiple hidden />
+        <input type="file" id="scene-img-input" accept="image/*" hidden />
         <ol id="playlist" class="playlist"></ol>
-        <div class="hint-mini">Clic sul <b>nome</b> = avvia/riavvia dall'inizio · ▶/⏸ pausa/riprendi · ⠿ trascina per riordinare · ⌨ tasto rapido</div>
+        <div class="hint-mini">Clic sul <b>nome</b> = avvia/riavvia · ▶/⏸ pausa · ⠿ riordina · ⌨ tasto rapido · 🎬 <b>scena</b> (effetto+testo+immagine applicati quando parte il brano)</div>
       </section>
 
       <section>