[FEAT] Layer bugfix and new demo video

src/App.vue

@@ -51,7 +51,7 @@
 
       <main class="flex flex-col flex-1 bg-white/90 dark:bg-gray-800/90 backdrop-blur-sm rounded-3xl shadow-xl border border-gray-200/50 dark:border-gray-700/50 lg:overflow-hidden transition-all duration-300">
         <!-- Welcome state -->
-        <div v-if="!visibleLayers.some(l => l.sprites.length)" class="p-6 sm:p-10 overflow-y-auto">
+        <div v-if="!layers.some(l => l.sprites.length)" class="p-6 sm:p-10 overflow-y-auto">
           <div class="mb-8">
             <h2 class="text-2xl font-bold text-gray-800 dark:text-gray-100 mb-1">Upload sprites or single image</h2>
             <p class="text-sm text-gray-600 dark:text-gray-400">Drag and drop images or import from JSON</p>
@@ -81,7 +81,7 @@
                     How it works
                   </h4>
                   <video controls playsinline class="w-full rounded-lg shadow-lg border border-gray-200 dark:border-gray-700" title="Spritesheet generator tutorial" aria-label="Spritesheet generator tutorial">
-                    <source src="@/assets/tut2.mp4" type="video/mp4" />
+                    <source src="@/assets/demo.mp4" type="video/mp4" />
                   </video>
                 </div>
               </div>
@@ -90,7 +90,7 @@
         </div>
 
         <!-- Two-column layout: Left controls, Right preview -->
-        <div v-if="visibleLayers.some(l => l.sprites.length)" class="flex flex-col flex-1 lg:grid lg:grid-cols-[380px_1fr] xl:grid-cols-[420px_1fr] lg:overflow-hidden">
+        <div v-if="layers.some(l => l.sprites.length)" class="flex flex-col flex-1 lg:grid lg:grid-cols-[380px_1fr] xl:grid-cols-[420px_1fr] lg:overflow-hidden">
           <!-- Left sidebar - Controls -->
           <div class="p-6 bg-gray-50/50 dark:bg-gray-900/30 border-b lg:border-b-0 lg:border-r border-gray-200 dark:border-gray-700 lg:overflow-y-auto lg:overflow-x-auto">
             <div class="space-y-6">