Add live LED strip preview via WebSocket on target cards

Stream real-time LED colors from running WLED targets to the browser via
binary WebSocket (RGB bytes, throttled to ~15 fps). Toggle button on
target card opens a compact canvas strip that renders each frame using
ImageData. Cached last frame is re-rendered after card reconciliation to
prevent flicker during auto-refresh.

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

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

@@ -587,3 +587,18 @@ ul.section-tip li {
         grid-template-columns: 1fr;
     }
 }
+
+/* ── LED Preview Panel ────────────────────────────────────────── */
+
+.led-preview-panel {
+    padding: 4px 0 0;
+}
+
+.led-preview-canvas {
+    display: block;
+    width: 100%;
+    height: 16px;
+    border-radius: 3px;
+    image-rendering: pixelated;
+    background: #111;
+}