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/zh.json

@@ -913,6 +913,10 @@
   "color_strip.notification.endpoint.hint": "使用此 URL 从外部系统触发通知。POST 请求可选 JSON：{\"app\": \"AppName\", \"color\": \"#FF0000\"}。",
   "color_strip.notification.save_first": "请先保存源以查看 Webhook 端点 URL。",
   "color_strip.notification.app_count": "个应用",
+  "color_strip.notification.test": "测试通知",
+  "color_strip.notification.test.ok": "通知已发送",
+  "color_strip.notification.test.no_streams": "此源没有运行中的流",
+  "color_strip.notification.test.error": "发送通知失败",
   "color_strip.type.daylight": "日光循环",
   "color_strip.type.daylight.desc": "模拟24小时自然日光变化",
   "color_strip.type.daylight.hint": "模拟太阳在24小时内的色温变化——从温暖的日出到冷白的日光，再到温暖的日落和昏暗的夜晚。",