add status indicator when adding a URL

ui/src/app/app.html

@@ -104,7 +104,15 @@
                 Canceling...
               </button>
             } @else if (addInProgress) {
-              <button class="btn btn-danger btn-lg px-3" type="button" (click)="cancelAdding()">
+              <button class="btn btn-secondary btn-lg px-3 add-progress-btn" type="button" disabled>
+                <span class="spinner-border spinner-border-sm me-2" role="status"></span>
+                Adding...
+              </button>
+              <button class="btn btn-outline-danger btn-lg px-3 add-cancel-btn"
+                type="button"
+                (click)="cancelAdding()"
+                aria-label="Cancel adding URL"
+                title="Cancel adding URL">
                 <fa-icon [icon]="faTimesCircle" class="me-1" /> Cancel
               </button>
             } @else {

ui/src/app/app.sass

@@ -112,6 +112,13 @@ td
   .spinner-border
     margin-right: 0.5rem
 
+.add-progress-btn
+  min-width: 9.5rem
+  cursor: default
+
+.add-cancel-btn
+  min-width: 3.25rem
+
 ::ng-deep .ng-select
     flex: 1
     .ng-select-container