styles: Set `background-repeat: no-repeat` for menu tabs

Images are used e.g. by the ProofreadPage extension.

Bug: T314028
Change-Id: I278fff4f02dd91effe5d8ace2f4a21a437a5114b

resources/skins.vector.styles/components/MenuTabs.less

@@ -44,6 +44,8 @@
 	// since they both appear beside each other in the article toolbar.
 	& > a,
 	.vector-menu-heading {
+		// Extensions like ProofreadPage insert images to the tabs, see T314028.
+		background-repeat: no-repeat;
 		display: inline-flex;
 		position: relative;
 		// Top & bottom padding to increase clickable area.