Center the +/- glyphs in the quantity stepper buttons

.stepperButton relied on the browser's default button content
centering, which the appearance: none reset in the previous commit
took away. Explicitly flex-center it (and zero out padding/line-height)
across all three copies (MenuItemCustomizer, MenuList, basket).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RvYdJ3os7tumimsLdLu2j9

src/components/ordering/MenuList.module.scss

@@ -75,12 +75,17 @@
 }
 
 .stepperButton {
+  display: flex;
+  align-items: center;
+  justify-content: center;
   width: 2rem;
   height: 2rem;
+  padding: 0;
   border-radius: 50%;
   border: 1px solid $color-border;
   background: $color-background;
   color: $color-text;
+  line-height: 1;
   cursor: pointer;
   align-self: center;
   justify-self: center;