Add PurchaseQuantitySelector for stackable product purchases.

Extract quantity controls and total amount display into a dedicated component with design-system inputs.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/dashboard/src/components/payments/purchase-quantity-selector.tsx

@@ -0,0 +1,99 @@
+"use client";
+
+import { DesignInput } from "@/components/design-components/input";
+import { DesignButton } from "@/components/design-components/button";
+import { Typography } from "@/components/ui";
+import { MinusIcon, PlusIcon } from "@phosphor-icons/react";
+import { shortenedInterval } from "./purchase-utils";
+
+type PriceData = {
+  USD?: string,
+  interval?: [number, string],
+};
+
+type Props = {
+  quantityInput: string,
+  quantityNumber: number,
+  onQuantityChange: (value: string) => void,
+  isTooLarge: boolean,
+  selectedPriceId: string,
+  priceData: PriceData,
+};
+
+export function PurchaseQuantitySelector({
+  quantityInput,
+  quantityNumber,
+  onQuantityChange,
+  isTooLarge,
+  selectedPriceId,
+  priceData,
+}: Props) {
+  return (
+    <div className="space-y-4">
+      <div className="space-y-3">
+        <div className="flex items-center justify-between gap-4">
+          <Typography type="label" className="text-sm font-semibold text-foreground">
+            Quantity
+          </Typography>
+          <div className="flex items-center gap-2">
+            <DesignButton
+              type="button"
+              size="icon"
+              variant="outline"
+              className="size-8 border-border/40 bg-foreground/[0.01] hover:bg-foreground/[0.03]"
+              disabled={quantityNumber <= 1}
+              onClick={() => onQuantityChange(String(Math.max(1, quantityNumber - 1)))}
+            >
+              <MinusIcon className="size-3.5 text-foreground" />
+            </DesignButton>
+            <DesignInput
+              className="h-8 w-20 text-center text-sm font-semibold tabular-nums border-border/40 bg-foreground/[0.01] text-foreground focus-visible:ring-blue-500/20"
+              inputMode="numeric"
+              pattern="[0-9]*"
+              type="text"
+              value={quantityInput}
+              onChange={(event) => {
+                const digitsOnly = event.target.value.replace(/[^0-9]/g, "");
+                onQuantityChange(digitsOnly);
+              }}
+            />
+            <DesignButton
+              type="button"
+              size="icon"
+              variant="outline"
+              className="size-8 border-border/40 bg-foreground/[0.01] hover:bg-foreground/[0.03]"
+              onClick={() => onQuantityChange(String(quantityNumber + 1))}
+            >
+              <PlusIcon className="size-3.5 text-foreground" />
+            </DesignButton>
+          </div>
+        </div>
+        {(quantityNumber < 1 || isTooLarge) && (
+          <Typography type="footnote" variant="destructive" className="text-xs">
+            {quantityNumber < 1
+              ? "Please enter a quantity of at least 1."
+              : "Amount exceeds the maximum limit of $999,999. Please reduce the quantity."}
+          </Typography>
+        )}
+      </div>
+
+      <div className="border-t border-border/40 pt-3">
+        <div className="flex items-baseline justify-between gap-3">
+          <Typography type="label" className="text-sm font-semibold text-foreground">
+            Total Amount
+          </Typography>
+          <div className="text-right">
+            <Typography type="h2" className="text-xl font-bold tabular-nums text-foreground">
+              ${selectedPriceId ? (Number(priceData.USD) * Math.max(0, quantityNumber)).toFixed(2) : "0.00"}
+            </Typography>
+            {selectedPriceId && priceData.interval && (
+              <Typography type="p" variant="secondary" className="text-xs text-muted-foreground mt-0.5">
+                per {shortenedInterval(priceData.interval)}
+              </Typography>
+            )}
+          </div>
+        </div>
+      </div>
+    </div>
+  );
+}