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/output.html

@@ -13,6 +13,9 @@
   <!-- Image overlay layer -->
   <div id="overlay-images"></div>
 
+  <!-- Per-track scene image -->
+  <img id="scene-image" class="scene-image" />
+
   <!-- Two independent, freely-positioned & resizable logos -->
   <img id="logo-0" class="logo" />
   <img id="logo-1" class="logo" />