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/js/app.js

@@ -125,7 +125,7 @@ import {
     onNotificationFilterModeChange,
     notificationAddAppColor, notificationRemoveAppColor,
     testNotification,
-    testColorStrip, closeTestCssSourceModal,
+    testColorStrip, closeTestCssSourceModal, applyCssTestLedCount, fireCssTestNotification, fireCssTestNotificationLayer,
 } from './features/color-strips.js';
 
 // Layer 5: audio sources
@@ -405,7 +405,7 @@ Object.assign(window, {
     onNotificationFilterModeChange,
     notificationAddAppColor, notificationRemoveAppColor,
     testNotification,
-    testColorStrip, closeTestCssSourceModal,
+    testColorStrip, closeTestCssSourceModal, applyCssTestLedCount, fireCssTestNotification, fireCssTestNotificationLayer,
 
     // audio sources
     showAudioSourceModal,