feat: Add purchase order flow with Orders pages

Add "Add to Order List" button on Results page that creates PurchaseItems
from optimization results and locks the job. Add Orders Index page with
tabbed view (Pending/Ordered/All), supplier assignment, status
transitions, and MaterialFilter. Add manual order item creation page.
Add Orders link to navigation menu.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CutList.Web/Components/Layout/NavMenu.razor

@@ -28,6 +28,11 @@
                 <span class="bi bi-boxes-nav-menu" aria-hidden="true"></span> Stock Items
             </NavLink>
         </div>
+        <div class="nav-item px-3">
+            <NavLink class="nav-link" href="orders">
+                <span class="bi bi-cart-nav-menu" aria-hidden="true"></span> Orders
+            </NavLink>
+        </div>
         <div class="nav-item px-3">
             <NavLink class="nav-link" href="suppliers">
                 <span class="bi bi-building-nav-menu" aria-hidden="true"></span> Suppliers