Add sales tracking system with customers, analytics, and inventory management

- Add customers table (021) and sales/sale_items tables (022) migrations
- Add customer CRUD, sale CRUD (transactional with auto inventory decrement/restore),
  and analytics API endpoints (overview, top sellers, revenue chart, inventory alerts)
- Add sales page with NewSaleModal (customer autocomplete, multi-item form,
  color-based pricing, stock validation) and SaleDetailModal
- Add customers page with search, inline editing, and purchase history
- Add analytics dashboard with recharts (revenue line chart, top sellers bar,
  refill vs spulna pie chart, inventory alerts table with stockout estimates)
- Add customerService, saleService, analyticsService to frontend API layer
- Update sidebar navigation on all admin pages

app/upadaj/requests/page.tsx

@@ -123,19 +123,12 @@ export default function ColorRequestsAdmin() {
       <div className="container mx-auto px-4 py-8">
         <div className="flex justify-between items-center mb-6">
           <h1 className="text-3xl font-bold text-gray-800 dark:text-gray-100">Zahtevi za Boje</h1>
-          <div className="space-x-4">
-            <Link
-              href="/dashboard"
-              className="px-4 py-2 bg-gray-600 text-white rounded hover:bg-gray-700"
-            >
-              Inventar
-            </Link>
-            <Link
-              href="/upadaj/colors"
-              className="px-4 py-2 bg-gray-600 text-white rounded hover:bg-gray-700"
-            >
-              Boje
-            </Link>
+          <div className="space-x-4 flex flex-wrap gap-2">
+            <Link href="/dashboard" className="px-4 py-2 bg-gray-600 text-white rounded hover:bg-gray-700">Inventar</Link>
+            <Link href="/upadaj/colors" className="px-4 py-2 bg-gray-600 text-white rounded hover:bg-gray-700">Boje</Link>
+            <Link href="/upadaj/sales" className="px-4 py-2 bg-gray-600 text-white rounded hover:bg-gray-700">Prodaja</Link>
+            <Link href="/upadaj/customers" className="px-4 py-2 bg-gray-600 text-white rounded hover:bg-gray-700">Kupci</Link>
+            <Link href="/upadaj/analytics" className="px-4 py-2 bg-gray-600 text-white rounded hover:bg-gray-700">Analitika</Link>
           </div>
         </div>