fix(files): add missing table header for actions column

- resolves #59333

Every column needs an header with some label for accessibility,
so screen reader know which column they are in currently.
This adds the missing header (but hides it visually so only for SR).

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

Author: susnux

apps/files/src/components/FilesListTableHeader.vue

@@ -28,7 +28,11 @@
 		</th>
 
 		<!-- Actions -->
-		<th class="files-list__row-actions" />
+		<th class="files-list__row-actions">
+			<span class="hidden-visually">
+				{{ t('files', 'Actions') }}
+			</span>
+		</th>
 
 		<!-- Mime -->
 		<th