Add per-layer LED preview for composite color strip sources

When a target uses a composite CSS source, the LED preview now shows
individual layer strips below the blended composite result. Backend
stores per-layer color snapshots and sends an extended binary wire
format; frontend renders separate canvases with hover labels.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/css/cards.css

@@ -935,3 +935,47 @@ ul.section-tip li {
 .led-preview-zones:hover .led-preview-zone-label {
     opacity: 1;
 }
+
+/* Per-layer LED preview (composite sources) */
+.led-preview-layers {
+    display: flex;
+    flex-direction: column;
+    gap: 2px;
+}
+
+.led-preview-layer {
+    position: relative;
+}
+
+.led-preview-layer-canvas {
+    display: block;
+    width: 100%;
+    height: 14px;
+    border-radius: 2px;
+    image-rendering: pixelated;
+    background: #111;
+}
+
+.led-preview-layer-composite .led-preview-layer-canvas {
+    height: 24px;
+    border-radius: 3px;
+}
+
+.led-preview-layer-label {
+    position: absolute;
+    left: 4px;
+    top: 50%;
+    transform: translateY(-50%);
+    font-size: 0.6rem;
+    font-family: var(--font-mono, monospace);
+    color: #fff;
+    text-shadow: 0 0 3px rgba(0,0,0,0.9), 0 0 6px rgba(0,0,0,0.6);
+    pointer-events: none;
+    opacity: 0;
+    transition: opacity 0.15s;
+    white-space: nowrap;
+}
+
+.led-preview-layers:hover .led-preview-layer-label {
+    opacity: 1;
+}