Add common.loading locale key and cancellable capture test overlay

Add missing common.loading i18n key to en/ru locales. Add close button
and ESC key support to the overlay spinner so users can cancel running
capture tests. Uses AbortController to abort the in-flight fetch request.

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

server/src/wled_controller/static/locales/ru.json

@@ -237,6 +237,7 @@
   "confirm.title": "Подтверждение Действия",
   "confirm.yes": "Да",
   "confirm.no": "Нет",
+  "common.loading": "Загрузка...",
   "common.delete": "Удалить",
   "common.edit": "Редактировать",
   "streams.title": "\uD83D\uDCFA Источники",