Add test button to notification CSS cards and fix header stability

- Bell icon button on notification source cards triggers POST /notify
- Shows success/warning/error toast based on response
- Fix header shifting when sticky by moving it outside .container
- i18n keys added for en, ru, zh

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

server/src/wled_controller/static/js/app.js

@@ -119,6 +119,7 @@ import {
     copyEndpointUrl,
     onNotificationFilterModeChange,
     notificationAddAppColor, notificationRemoveAppColor,
+    testNotification,
 } from './features/color-strips.js';
 
 // Layer 5: audio sources
@@ -391,6 +392,7 @@ Object.assign(window, {
     copyEndpointUrl,
     onNotificationFilterModeChange,
     notificationAddAppColor, notificationRemoveAppColor,
+    testNotification,
 
     // audio sources
     showAudioSourceModal,