Add configurable FPS to test preview and fix composite stream release race

- Add FPS control (1-60, default 20) to test preview modal next to LED count
- Server accepts fps query param, controls frame send interval
- Single Apply icon button (✓) applies both LED count and FPS
- FPS control stays visible for picture sources (LED count hidden)
- Fix composite sub-stream consumer ID collision: use unique instance ID
  to prevent old WebSocket release from killing new connection's streams

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