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/locales/en.json

@@ -913,6 +913,10 @@
   "color_strip.notification.endpoint.hint": "Use this URL to trigger notifications from external systems. POST with optional JSON body: {\"app\": \"AppName\", \"color\": \"#FF0000\"}.",
   "color_strip.notification.save_first": "Save the source first to see the webhook endpoint URL.",
   "color_strip.notification.app_count": "apps",
+  "color_strip.notification.test": "Test notification",
+  "color_strip.notification.test.ok": "Notification sent",
+  "color_strip.notification.test.no_streams": "No running streams for this source",
+  "color_strip.notification.test.error": "Failed to send notification",
   "color_strip.type.daylight": "Daylight Cycle",
   "color_strip.type.daylight.desc": "Simulates natural daylight over 24 hours",
   "color_strip.type.daylight.hint": "Simulates the sun's color temperature throughout a 24-hour day/night cycle — from warm sunrise to cool daylight to warm sunset and dim night.",