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/style.css

@@ -1006,6 +1006,24 @@ input:-webkit-autofill:focus {
     font-weight: 500;
 }
 
+.overlay-spinner-close {
+    position: absolute;
+    top: 16px;
+    right: 16px;
+    background: none;
+    border: none;
+    color: rgba(255, 255, 255, 0.6);
+    font-size: 32px;
+    cursor: pointer;
+    line-height: 1;
+    padding: 4px 8px;
+    transition: color 0.15s;
+}
+
+.overlay-spinner-close:hover {
+    color: white;
+}
+
 @keyframes spin {
     to { transform: rotate(360deg); }
 }