fix(cart): pad RelatedProducts to 3 cards even on first pick

The catalog only has 4 products; with 2 already in the cart, the
initial pick's pool shrinks to 2 and pickRandom's slice can't pad
past what's available, so the grid rendered 2 cards instead of 3.
The swap-after-add path already had fallback logic for this exact
case — extracted it into pickWithFallback() and reused it for the
initial pick too.

app/cart/components/RelatedProducts.tsx

@@ -16,6 +16,27 @@ function pickRandom(allIds: string[], excludeIds: string[], count: number): stri
   return shuffled.slice(0, count);
 }
 
+// The catalog only has a handful of products — once the cart holds enough
+// distinct ones, "N recommendations that aren't already in the cart" can
+// become impossible (e.g. 4 products total, 2 already in the cart, but
+// DISPLAY_COUNT is 3 — only 2 non-cart products exist, period). Falling
+// back to re-suggesting something already in the cart (a normal "grab
+// another one" pattern) beats silently shrinking the grid below
+// DISPLAY_COUNT. Shared by both the initial pick and the swap-after-add
+// path so neither can under-fill the grid.
+function pickWithFallback(allIds: string[], excludeIds: string[], keep: string[], count: number): string[] {
+  const missing = count - keep.length;
+  if (missing <= 0) return keep;
+
+  let picks = pickRandom(allIds, [...excludeIds, ...keep], missing);
+  if (picks.length < missing) {
+    const stillMissing = missing - picks.length;
+    const fallback = pickRandom(allIds, [...keep, ...picks], stillMissing);
+    picks = [...picks, ...fallback];
+  }
+  return [...keep, ...picks];
+}
+
 export function RelatedProducts() {
   const cart = useCart();
   const products = useProducts();
@@ -55,7 +76,7 @@ export function RelatedProducts() {
 
     if (!pickedRef.current) {
       pickedRef.current = true;
-      setDisplayIds(pickRandom(productIds, cartIds, DISPLAY_COUNT));
+      setDisplayIds(pickWithFallback(productIds, cartIds, [], DISPLAY_COUNT));
       return;
     }
 
@@ -67,24 +88,7 @@ export function RelatedProducts() {
     swapTimeoutRef.current = setTimeout(() => {
       setDisplayIds((prev) => {
         const stillRelevant = prev.filter((id) => !cartIds.includes(id));
-        const missing = DISPLAY_COUNT - stillRelevant.length;
-        if (missing <= 0) return stillRelevant;
-
-        let replacements = pickRandom(productIds, [...cartIds, ...stillRelevant], missing);
-
-        // The catalog only has a handful of products — once the cart
-        // holds enough distinct ones, "N recommendations that aren't
-        // already in the cart" can become impossible. Falling back to
-        // re-suggesting something already in the cart (a normal "grab
-        // another one" pattern) beats silently shrinking the grid below
-        // DISPLAY_COUNT.
-        if (replacements.length < missing) {
-          const stillMissing = missing - replacements.length;
-          const fallback = pickRandom(productIds, [...stillRelevant, ...replacements], stillMissing);
-          replacements = [...replacements, ...fallback];
-        }
-
-        return [...stillRelevant, ...replacements];
+        return pickWithFallback(productIds, cartIds, stillRelevant, DISPLAY_COUNT);
       });
     }, FEEDBACK_MS);
   }, [cartKey, productIds]);