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/colors/page.tsx

@@ -193,18 +193,12 @@ export default function ColorsManagement() {
           <div className="p-6">
             <h2 className="text-xl font-bold text-gray-900 dark:text-white mb-6">Admin Panel</h2>
             <nav className="space-y-2">
-              <a
-                href="/dashboard"
-                className="block px-4 py-2 text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-700 rounded"
-              >
-                Filamenti
-              </a>
-              <a
-                href="/upadaj/colors"
-                className="block px-4 py-2 bg-blue-100 dark:bg-blue-900 text-blue-700 dark:text-blue-300 rounded"
-              >
-                Boje
-              </a>
+              <a href="/dashboard" className="block px-4 py-2 text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-700 rounded">Filamenti</a>
+              <a href="/upadaj/colors" className="block px-4 py-2 bg-blue-100 dark:bg-blue-900 text-blue-700 dark:text-blue-300 rounded">Boje</a>
+              <a href="/upadaj/requests" className="block px-4 py-2 text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-700 rounded">Zahtevi za boje</a>
+              <a href="/upadaj/sales" className="block px-4 py-2 text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-700 rounded">Prodaja</a>
+              <a href="/upadaj/customers" className="block px-4 py-2 text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-700 rounded">Kupci</a>
+              <a href="/upadaj/analytics" className="block px-4 py-2 text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-700 rounded">Analitika</a>
             </nav>
           </div>
         </div>