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/en.json

@@ -237,6 +237,7 @@
   "confirm.title": "Confirm Action",
   "confirm.yes": "Yes",
   "confirm.no": "No",
+  "common.loading": "Loading...",
   "common.delete": "Delete",
   "common.edit": "Edit",
   "streams.title": "\uD83D\uDCFA Sources",