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/components/ordering/MenuList.tsx

@@ -1,3 +1,4 @@
+import { AnimatedNumber } from '#/components/ui/AnimatedNumber.tsx'
 import type { MenuItemWithRecipe } from '#/domain/ordering.ts'
 
 import styles from './MenuList.module.scss'
@@ -51,7 +52,7 @@ export const MenuList = ({
             >
               −
             </button>
-            <span className={styles.count}>{quantity}</span>
+            <AnimatedNumber value={quantity} className={styles.count} />
             <button
               type="button"
               className={styles.stepperButton}

src/components/ui/AnimatedNumber.module.scss

@@ -0,0 +1,11 @@
+.wrapper {
+  position: relative;
+  display: inline-grid;
+  justify-items: center;
+  overflow: hidden;
+  vertical-align: middle;
+}
+
+.digit {
+  grid-area: 1 / 1;
+}

src/components/ui/AnimatedNumber.tsx

@@ -0,0 +1,58 @@
+import { AnimatePresence, motion, useReducedMotion } from 'framer-motion'
+import { useState } from 'react'
+
+import styles from './AnimatedNumber.module.scss'
+
+type AnimatedNumberProps = {
+  value: number
+  className?: string
+}
+
+const variants = {
+  initial: (direction: number) => ({ x: direction * 12, opacity: 0 }),
+  animate: { x: 0, opacity: 1 },
+  exit: (direction: number) => ({ x: direction * -12, opacity: 0 }),
+}
+
+// A quantity stepper's count slides leftward when incrementing (the new
+// digit enters from the right, the old one exits to the left) and
+// rightward when decrementing.
+//
+// `direction` is derived from a prop change during render (the React-docs
+// "adjusting state when a prop changes" pattern) rather than mutating a ref
+// in the render body — mutating a ref here would already reflect the new
+// value by the time `exit`'s "custom" is resolved for the outgoing digit,
+// so every transition — increment or decrement — read as "forward".
+export const AnimatedNumber = ({ value, className }: AnimatedNumberProps) => {
+  const reduceMotion = useReducedMotion()
+  const [previousValue, setPreviousValue] = useState(value)
+  const [direction, setDirection] = useState(1)
+
+  if (value !== previousValue) {
+    setDirection(value > previousValue ? 1 : -1)
+    setPreviousValue(value)
+  }
+
+  if (reduceMotion) {
+    return <span className={className}>{value}</span>
+  }
+
+  return (
+    <span className={`${styles.wrapper} ${className ?? ''}`}>
+      <AnimatePresence mode="popLayout" initial={false} custom={direction}>
+        <motion.span
+          key={value}
+          custom={direction}
+          variants={variants}
+          initial="initial"
+          animate="animate"
+          exit="exit"
+          transition={{ duration: 0.15, ease: 'easeOut' }}
+          className={styles.digit}
+        >
+          {value}
+        </motion.span>
+      </AnimatePresence>
+    </span>
+  )
+}

src/routes/__root.module.scss

@@ -1,7 +1,8 @@
 @use '../styles/variables' as *;
 
 .topBar {
-  position: relative;
+  position: sticky;
+  top: 0;
   // Explicit stacking context, ranked above any page content: setting
   // `view-transition-name` on an element (here and on OrderStatusView's
   // order card) implicitly makes it its own stacking context too — without
@@ -13,6 +14,7 @@
   justify-content: space-between;
   align-items: center;
   padding: $space-3;
+  background: $color-background;
   border-bottom: 1px solid $color-border;
 }
 

src/routes/basket.tsx

@@ -1,6 +1,7 @@
 import { createFileRoute, Link, useNavigate } from '@tanstack/react-router'
 
 import { CheckoutSection } from '#/components/ordering/CheckoutSection.tsx'
+import { AnimatedNumber } from '#/components/ui/AnimatedNumber.tsx'
 import { useBasket } from '#/lib/basket.tsx'
 import { getBrand } from '#/server/brand.ts'
 import { getMenu } from '#/server/menu.ts'
@@ -54,7 +55,7 @@ const BasketPage = () => {
                   >
                     −
                   </button>
-                  <span className={styles.count}>{quantity}</span>
+                  <AnimatedNumber value={quantity} className={styles.count} />
                   <button
                     type="button"
                     className={styles.stepperButton}

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: