fix: unify free shipping threshold to 10000 RSD across all pages

- Create centralized shipping config file (src/lib/config/shipping.ts)
- Update CartDrawer.tsx: change hardcoded 5000 to FREE_SHIPPING_THRESHOLD_RSD constant
- Update TickerBar.tsx: change hardcoded 10000 to use centralized constant
- Ensure uniform free shipping messaging across all pages and localizations

src/lib/config/shipping.ts

@@ -0,0 +1,50 @@
+/**
+ * Shipping configuration
+ * Centralized configuration for shipping rules and thresholds
+ */
+
+/**
+ * Free shipping threshold in RSD (Serbian Dinar)
+ * Orders above this amount qualify for free shipping
+ */
+export const FREE_SHIPPING_THRESHOLD_RSD = 10000;
+
+/**
+ * Default shipping cost in RSD when order is below threshold
+ */
+export const DEFAULT_SHIPPING_COST_RSD = 500;
+
+/**
+ * Currency code for shipping calculations
+ */
+export const SHIPPING_CURRENCY = "RSD";
+
+/**
+ * Check if an order qualifies for free shipping
+ * @param orderTotal - The total amount of the order
+ * @returns boolean indicating if order qualifies for free shipping
+ */
+export function qualifiesForFreeShipping(orderTotal: number): boolean {
+  return orderTotal >= FREE_SHIPPING_THRESHOLD_RSD;
+}
+
+/**
+ * Calculate shipping cost based on order total
+ * @param orderTotal - The total amount of the order
+ * @returns Shipping cost (0 if qualifies for free shipping)
+ */
+export function calculateShippingCost(orderTotal: number): number {
+  return qualifiesForFreeShipping(orderTotal) ? 0 : DEFAULT_SHIPPING_COST_RSD;
+}
+
+/**
+ * Get the remaining amount needed for free shipping
+ * @param orderTotal - The current order total
+ * @returns Amount needed to reach free shipping threshold (0 if already qualified)
+ */
+export function getRemainingForFreeShipping(orderTotal: number): number {
+  if (qualifiesForFreeShipping(orderTotal)) {
+    return 0;
+  }
+  return FREE_SHIPPING_THRESHOLD_RSD - orderTotal;
+}