feat: implement locale-aware routing with [locale] dynamic segments

WARNING: This change breaks existing SEO URLs for Serbian locale.

Changes:
- Migrated from separate locale folders (src/app/en/, src/app/de/, etc.)
  to [locale] dynamic segments (src/app/[locale]/)
- Serbian is now at /sr/ instead of / (root)
- English at /en/, German at /de/, French at /fr/
- All components updated to generate locale-aware links
- Root / now redirects to /sr (307 temporary redirect)

SEO Impact:
- Previously indexed Serbian URLs (/, /products, /about, /contact)
  will now return 404 or redirect to /sr/* URLs
- This is a breaking change for SEO - Serbian pages should ideally
  remain at root (/) with only non-default locales getting prefix
- Consider implementing 301 redirects from old URLs to maintain
  search engine rankings

Technical Notes:
- next-intl v4 with [locale] structure requires ALL locales to
  have the prefix (cannot have default locale at root)
- Alternative approach would be separate folder structure per locale

src/app/[locale]/checkout/page.tsx

@@ -0,0 +1,384 @@
+"use client";
+
+import { useState, useEffect } from "react";
+import { useRouter } from "next/navigation";
+import Link from "next/link";
+import Image from "next/image";
+import { useTranslations, useLocale } from "next-intl";
+import Header from "@/components/layout/Header";
+import Footer from "@/components/layout/Footer";
+import { useSaleorCheckoutStore } from "@/stores/saleorCheckoutStore";
+import { formatPrice } from "@/lib/saleor";
+import { saleorClient } from "@/lib/saleor/client";
+import {
+  CHECKOUT_SHIPPING_ADDRESS_UPDATE,
+  CHECKOUT_BILLING_ADDRESS_UPDATE,
+  CHECKOUT_COMPLETE,
+} from "@/lib/saleor/mutations/Checkout";
+import type { Checkout } from "@/types/saleor";
+
+interface ShippingAddressUpdateResponse {
+  checkoutShippingAddressUpdate?: {
+    checkout?: Checkout;
+    errors?: Array<{ message: string }>;
+  };
+}
+
+interface BillingAddressUpdateResponse {
+  checkoutBillingAddressUpdate?: {
+    checkout?: Checkout;
+    errors?: Array<{ message: string }>;
+  };
+}
+
+interface CheckoutCompleteResponse {
+  checkoutComplete?: {
+    order?: { number: string };
+    errors?: Array<{ message: string }>;
+  };
+}
+
+interface AddressForm {
+  firstName: string;
+  lastName: string;
+  streetAddress1: string;
+  streetAddress2: string;
+  city: string;
+  postalCode: string;
+  phone: string;
+}
+
+export default function CheckoutPage() {
+  const t = useTranslations("Checkout");
+  const locale = useLocale();
+  const router = useRouter();
+  const { checkout, refreshCheckout, getLines, getTotal } = useSaleorCheckoutStore();
+  const [isLoading, setIsLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+  const [orderComplete, setOrderComplete] = useState(false);
+  const [orderNumber, setOrderNumber] = useState<string | null>(null);
+
+  const [sameAsShipping, setSameAsShipping] = useState(true);
+  const [shippingAddress, setShippingAddress] = useState<AddressForm>({
+    firstName: "",
+    lastName: "",
+    streetAddress1: "",
+    streetAddress2: "",
+    city: "",
+    postalCode: "",
+    phone: "",
+  });
+  const [billingAddress, setBillingAddress] = useState<AddressForm>({
+    firstName: "",
+    lastName: "",
+    streetAddress1: "",
+    streetAddress2: "",
+    city: "",
+    postalCode: "",
+    phone: "",
+  });
+
+  const lines = getLines();
+  const total = getTotal();
+
+  useEffect(() => {
+    if (!checkout) {
+      refreshCheckout();
+    }
+  }, [checkout, refreshCheckout]);
+
+  const handleShippingChange = (field: keyof AddressForm, value: string) => {
+    setShippingAddress((prev) => ({ ...prev, [field]: value }));
+    if (sameAsShipping) {
+      setBillingAddress((prev) => ({ ...prev, [field]: value }));
+    }
+  };
+
+  const handleBillingChange = (field: keyof AddressForm, value: string) => {
+    setBillingAddress((prev) => ({ ...prev, [field]: value }));
+  };
+
+  const handleSubmit = async (e: React.FormEvent) => {
+    e.preventDefault();
+
+    if (!checkout) {
+      setError(t("errorNoCheckout"));
+      return;
+    }
+
+    setIsLoading(true);
+    setError(null);
+
+    try {
+      const shippingResult = await saleorClient.mutate<ShippingAddressUpdateResponse>({
+        mutation: CHECKOUT_SHIPPING_ADDRESS_UPDATE,
+        variables: {
+          checkoutId: checkout.id,
+          shippingAddress: {
+            ...shippingAddress,
+            country: "RS",
+          },
+        },
+      });
+
+      if (shippingResult.data?.checkoutShippingAddressUpdate?.errors && shippingResult.data.checkoutShippingAddressUpdate.errors.length > 0) {
+        throw new Error(shippingResult.data.checkoutShippingAddressUpdate.errors[0].message);
+      }
+
+      const billingResult = await saleorClient.mutate<BillingAddressUpdateResponse>({
+        mutation: CHECKOUT_BILLING_ADDRESS_UPDATE,
+        variables: {
+          checkoutId: checkout.id,
+          billingAddress: {
+            ...billingAddress,
+            country: "RS",
+          },
+        },
+      });
+
+      if (billingResult.data?.checkoutBillingAddressUpdate?.errors && billingResult.data.checkoutBillingAddressUpdate.errors.length > 0) {
+        throw new Error(billingResult.data.checkoutBillingAddressUpdate.errors[0].message);
+      }
+
+      const completeResult = await saleorClient.mutate<CheckoutCompleteResponse>({
+        mutation: CHECKOUT_COMPLETE,
+        variables: {
+          checkoutId: checkout.id,
+        },
+      });
+
+      if (completeResult.data?.checkoutComplete?.errors && completeResult.data.checkoutComplete.errors.length > 0) {
+        throw new Error(completeResult.data.checkoutComplete.errors[0].message);
+      }
+
+      const order = completeResult.data?.checkoutComplete?.order;
+      if (order) {
+        setOrderNumber(order.number);
+        setOrderComplete(true);
+      } else {
+        throw new Error(t("errorCreatingOrder"));
+      }
+    } catch (err: unknown) {
+      const errorMessage = err instanceof Error ? err.message : null;
+      setError(errorMessage || t("errorOccurred"));
+    } finally {
+      setIsLoading(false);
+    }
+  };
+
+  if (orderComplete) {
+    return (
+      <>
+        <Header locale={locale} />
+        <main className="min-h-screen">
+          <section className="pt-[120px] pb-20 px-4">
+            <div className="max-w-2xl mx-auto text-center">
+              <div className="mb-6">
+                <div className="w-16 h-16 bg-green-100 rounded-full flex items-center justify-center mx-auto mb-4">
+                  <svg className="w-8 h-8 text-green-600" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                    <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M5 13l4 4L19 7" />
+                  </svg>
+                </div>
+                <h1 className="text-3xl font-serif mb-2">{t("orderConfirmed")}</h1>
+                <p className="text-foreground-muted">{t("thankYou")}</p>
+              </div>
+
+              {orderNumber && (
+                <div className="bg-background-ice p-6 rounded-lg mb-6">
+                  <p className="text-sm text-foreground-muted mb-1">{t("orderNumber")}</p>
+                  <p className="text-2xl font-serif">#{orderNumber}</p>
+                </div>
+              )}
+
+              <p className="text-foreground-muted mb-8">
+                {t("confirmationEmail")}
+              </p>
+
+              <Link
+                href={`/${locale}/products`}
+                className="inline-block px-8 py-3 bg-foreground text-white hover:bg-accent-dark transition-colors"
+              >
+                {t("continueShoppingBtn")}
+              </Link>
+            </div>
+          </section>
+        </main>
+        <div className="pt-16">
+          <Footer locale={locale} />
+        </div>
+      </>
+    );
+  }
+
+  return (
+    <>
+      <Header />
+      <main className="min-h-screen">
+        <section className="pt-[120px] pb-20 px-4">
+          <div className="max-w-7xl mx-auto">
+            <h1 className="text-3xl font-serif mb-8">{t("checkout")}</h1>
+
+            {error && (
+              <div className="bg-red-50 border border-red-200 text-red-600 p-4 mb-6 rounded">
+                {error}
+              </div>
+            )}
+
+            <div className="grid grid-cols-1 lg:grid-cols-2 gap-12">
+              <div>
+                <form onSubmit={handleSubmit} className="space-y-6">
+                  <div className="border-b border-border pb-6">
+                    <h2 className="text-xl font-serif mb-4">{t("shippingAddress")}</h2>
+                    <div className="grid grid-cols-2 gap-4">
+                      <div>
+                        <label className="block text-sm font-medium mb-1">{t("firstName")}</label>
+                        <input
+                          type="text"
+                          required
+                          value={shippingAddress.firstName}
+                          onChange={(e) => handleShippingChange("firstName", e.target.value)}
+                          className="w-full border border-border px-4 py-2 rounded"
+                        />
+                      </div>
+                      <div>
+                        <label className="block text-sm font-medium mb-1">{t("lastName")}</label>
+                        <input
+                          type="text"
+                          required
+                          value={shippingAddress.lastName}
+                          onChange={(e) => handleShippingChange("lastName", e.target.value)}
+                          className="w-full border border-border px-4 py-2 rounded"
+                        />
+                      </div>
+                      <div className="col-span-2">
+                        <label className="block text-sm font-medium mb-1">{t("streetAddress")}</label>
+                        <input
+                          type="text"
+                          required
+                          value={shippingAddress.streetAddress1}
+                          onChange={(e) => handleShippingChange("streetAddress1", e.target.value)}
+                          className="w-full border border-border px-4 py-2 rounded"
+                        />
+                      </div>
+                      <div className="col-span-2">
+                        <input
+                          type="text"
+                          value={shippingAddress.streetAddress2}
+                          onChange={(e) => handleShippingChange("streetAddress2", e.target.value)}
+                          placeholder={t("streetAddressOptional")}
+                          className="w-full border border-border px-4 py-2 rounded"
+                        />
+                      </div>
+                      <div>
+                        <label className="block text-sm font-medium mb-1">{t("city")}</label>
+                        <input
+                          type="text"
+                          required
+                          value={shippingAddress.city}
+                          onChange={(e) => handleShippingChange("city", e.target.value)}
+                          className="w-full border border-border px-4 py-2 rounded"
+                        />
+                      </div>
+                      <div>
+                        <label className="block text-sm font-medium mb-1">{t("postalCode")}</label>
+                        <input
+                          type="text"
+                          required
+                          value={shippingAddress.postalCode}
+                          onChange={(e) => handleShippingChange("postalCode", e.target.value)}
+                          className="w-full border border-border px-4 py-2 rounded"
+                        />
+                      </div>
+                      <div className="col-span-2">
+                        <label className="block text-sm font-medium mb-1">{t("phone")}</label>
+                        <input
+                          type="tel"
+                          required
+                          value={shippingAddress.phone}
+                          onChange={(e) => handleShippingChange("phone", e.target.value)}
+                          className="w-full border border-border px-4 py-2 rounded"
+                        />
+                      </div>
+                    </div>
+                  </div>
+
+                  <div className="border-b border-border pb-6">
+                    <label className="flex items-center gap-2 cursor-pointer">
+                      <input
+                        type="checkbox"
+                        checked={sameAsShipping}
+                        onChange={(e) => setSameAsShipping(e.target.checked)}
+                        className="w-4 h-4"
+                      />
+                      <span>{t("billingAddressSame")}</span>
+                    </label>
+                  </div>
+
+                  <button
+                    type="submit"
+                    disabled={isLoading || lines.length === 0}
+                    className="w-full py-4 bg-foreground text-white font-medium hover:bg-accent-dark transition-colors disabled:opacity-50"
+                  >
+                    {isLoading ? t("processing") : t("completeOrder", { total: formatPrice(total) })}
+                  </button>
+                </form>
+              </div>
+
+              <div className="bg-background-ice p-6 rounded-lg h-fit">
+                <h2 className="text-xl font-serif mb-6">{t("orderSummary")}</h2>
+
+                {lines.length === 0 ? (
+                  <p className="text-foreground-muted">{t("yourCartEmpty")}</p>
+                ) : (
+                  <>
+                    <div className="space-y-4 mb-6">
+                      {lines.map((line) => (
+                        <div key={line.id} className="flex gap-4">
+                          <div className="w-16 h-16 bg-white relative flex-shrink-0">
+                            {line.variant.product.media[0]?.url && (
+                              <Image
+                                src={line.variant.product.media[0].url}
+                                alt={line.variant.product.name}
+                                fill
+                                sizes="64px"
+                                className="object-cover"
+                              />
+                            )}
+                          </div>
+                          <div className="flex-1">
+                            <h3 className="font-medium text-sm">{line.variant.product.name}</h3>
+                            <p className="text-foreground-muted text-sm">
+                              {t("qty")}: {line.quantity}
+                            </p>
+                            <p className="text-sm">
+                              {formatPrice(line.totalPrice.gross.amount)}
+                            </p>
+                          </div>
+                        </div>
+                      ))}
+                    </div>
+
+                    <div className="border-t border-border pt-4 space-y-2">
+                      <div className="flex justify-between">
+                        <span className="text-foreground-muted">{t("subtotal")}</span>
+                        <span>{formatPrice(checkout?.subtotalPrice?.gross?.amount || 0)}</span>
+                      </div>
+                      <div className="flex justify-between font-medium text-lg pt-2 border-t border-border">
+                        <span>{t("total")}</span>
+                        <span>{formatPrice(total)}</span>
+                      </div>
+                    </div>
+                  </>
+                )}
+              </div>
+            </div>
+          </div>
+        </section>
+      </main>
+
+      <div className="pt-16">
+        <Footer />
+      </div>
+    </>
+  );
+}