Add composite layer preview, configurable LED count, and notification fire button to CSS test modal

- Composite sources show per-layer strip canvases with composite result on top
- Server sends composite wire format with per-layer RGB data
- LED count is configurable via input field, persisted in localStorage
- Notification sources show a bell fire button on the strip preview
- Composite with notification layers shows per-layer fire buttons
- Fixed stale WS frame bug with generation counter and unique consumer IDs
- Modal width is now fixed at 700px to prevent layout jumps
- Target card composite layers now use same-height canvases

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

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

@@ -1052,15 +1052,10 @@ ul.section-tip li {
 .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;
+    image-rendering: pixelated;
+    background: #111;
 }
 
 .led-preview-layer-label {