refactor: centralize locale constants to prevent breaking changes

Created src/lib/i18n/locales.ts as single source of truth for:
- SUPPORTED_LOCALES array
- LOCALE_COOKIE name
- DEFAULT_LOCALE
- LOCALE_CONFIG (labels, flags, Saleor locale mapping)
- Helper functions (isValidLocale, getSaleorLocale, getLocaleFromPath)

Updated all files to use centralized constants:
- middleware.ts
- Header.tsx
- ProductCard.tsx
- sitemap.ts
- root layout and locale layout
- routing.ts

Benefits:
- Adding new locale only requires updating ONE file (locales.ts)
- No more hardcoded locale lists scattered across codebase
- Cookie name defined in one place
- Type-safe locale validation

src/app/sitemap.ts

@@ -1,7 +1,7 @@
 import { MetadataRoute } from "next";
 import { getProducts } from "@/lib/saleor";
+import { SUPPORTED_LOCALES, type Locale } from "@/lib/i18n/locales";
 
-const LOCALES = ["sr", "en", "de", "fr"] as const;
 const baseUrl = process.env.NEXT_PUBLIC_SITE_URL || "https://dev.manoonoils.com";
 
 interface SitemapEntry {
@@ -29,12 +29,9 @@ export default async function sitemap(): Promise<SitemapEntry[]> {
       changeFrequency: "daily",
       priority: 1,
       alternates: {
-        languages: {
-          sr: `${baseUrl}`,
-          en: `${baseUrl}/en`,
-          de: `${baseUrl}/de`,
-          fr: `${baseUrl}/fr`,
-        },
+        languages: Object.fromEntries(
+          SUPPORTED_LOCALES.map((locale) => [locale, locale === "sr" ? baseUrl : `${baseUrl}/${locale}`])
+        ),
       },
     },
     {
@@ -43,12 +40,9 @@ export default async function sitemap(): Promise<SitemapEntry[]> {
       changeFrequency: "daily",
       priority: 0.9,
       alternates: {
-        languages: {
-          sr: `${baseUrl}/products`,
-          en: `${baseUrl}/en/products`,
-          de: `${baseUrl}/de/products`,
-          fr: `${baseUrl}/fr/products`,
-        },
+        languages: Object.fromEntries(
+          SUPPORTED_LOCALES.map((locale) => [locale, locale === "sr" ? `${baseUrl}/products` : `${baseUrl}/${locale}/products`])
+        ),
       },
     },
     {
@@ -57,12 +51,9 @@ export default async function sitemap(): Promise<SitemapEntry[]> {
       changeFrequency: "monthly",
       priority: 0.6,
       alternates: {
-        languages: {
-          sr: `${baseUrl}/about`,
-          en: `${baseUrl}/en/about`,
-          de: `${baseUrl}/de/about`,
-          fr: `${baseUrl}/fr/about`,
-        },
+        languages: Object.fromEntries(
+          SUPPORTED_LOCALES.map((locale) => [locale, locale === "sr" ? `${baseUrl}/about` : `${baseUrl}/${locale}/about`])
+        ),
       },
     },
     {
@@ -71,12 +62,9 @@ export default async function sitemap(): Promise<SitemapEntry[]> {
       changeFrequency: "monthly",
       priority: 0.6,
       alternates: {
-        languages: {
-          sr: `${baseUrl}/contact`,
-          en: `${baseUrl}/en/contact`,
-          de: `${baseUrl}/de/contact`,
-          fr: `${baseUrl}/fr/contact`,
-        },
+        languages: Object.fromEntries(
+          SUPPORTED_LOCALES.map((locale) => [locale, locale === "sr" ? `${baseUrl}/contact` : `${baseUrl}/${locale}/contact`])
+        ),
       },
     },
     {
@@ -85,12 +73,9 @@ export default async function sitemap(): Promise<SitemapEntry[]> {
       changeFrequency: "monthly",
       priority: 0.5,
       alternates: {
-        languages: {
-          sr: `${baseUrl}/checkout`,
-          en: `${baseUrl}/en/checkout`,
-          de: `${baseUrl}/de/checkout`,
-          fr: `${baseUrl}/fr/checkout`,
-        },
+        languages: Object.fromEntries(
+          SUPPORTED_LOCALES.map((locale) => [locale, locale === "sr" ? `${baseUrl}/checkout` : `${baseUrl}/${locale}/checkout`])
+        ),
       },
     },
   ];
@@ -98,7 +83,13 @@ export default async function sitemap(): Promise<SitemapEntry[]> {
   const productUrls: SitemapEntry[] = [];
 
   for (const product of products) {
-    for (const locale of LOCALES) {
+    const hreflangs: Record<string, string> = {};
+    for (const locale of SUPPORTED_LOCALES) {
+      const path = locale === "sr" ? `/products/${product.slug}` : `/${locale}/products/${product.slug}`;
+      hreflangs[locale] = `${baseUrl}${path}`;
+    }
+
+    for (const locale of SUPPORTED_LOCALES) {
       const localePrefix = locale === "sr" ? "" : `/${locale}`;
       productUrls.push({
         url: `${baseUrl}${localePrefix}/products/${product.slug}`,
@@ -106,12 +97,7 @@ export default async function sitemap(): Promise<SitemapEntry[]> {
         changeFrequency: "weekly",
         priority: 0.8,
         alternates: {
-          languages: {
-            sr: `${baseUrl}/products/${product.slug}`,
-            en: `${baseUrl}/en/products/${product.slug}`,
-            de: `${baseUrl}/de/products/${product.slug}`,
-            fr: `${baseUrl}/fr/products/${product.slug}`,
-          },
+          languages: hreflangs,
         },
       });
     }