Animate quantity steppers and make the nav bar sticky

- Add an AnimatedNumber component that slides a quantity stepper's
  count horizontally — left on increment, right on decrement — and
  wire it into both the menu and basket steppers.
- Make the top nav bar position: sticky, with an explicit background
  so scrolled content doesn't show through it.
- Reserve the scrollbar's gutter on every page (scrollable or not) so
  the sticky header doesn't shift width when navigating between short
  and tall pages.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/styles/globals.scss

@@ -6,6 +6,16 @@
   box-sizing: border-box;
 }
 
+// Reserve the scrollbar's width on every page, scrollable or not — without
+// this, the sticky nav bar (and page content) shifts a few pixels wider
+// when navigating from a short page (no scrollbar) to a tall one (has one).
+// `scrollbar-gutter` only takes effect on an actual scroll container, so
+// `html` needs an explicit non-visible overflow for it to apply at all.
+html {
+  overflow-y: scroll;
+  scrollbar-gutter: stable;
+}
+
 body {
   margin: 0;
   font-family: