Add IsIncoming inventory flag and catalog-to-incoming powder flow in item wizard

- InventoryItem.IsIncoming: marks powder ordered but not yet received; enables QR code
  printing on work orders while the shipment is in transit
- InventoryController.CreateIncomingFromCatalog: POST endpoint creates a 0-balance inventory
  record from a PowderCatalogItem and returns it in wizard-compatible shape
- item-wizard.js: custom coat tab now searches the platform powder catalog as a fallback;
  catalog results show an 'Add as Incoming Order' option; createIncomingFromCatalog POSTs
  to server and selects the new item without a page refresh
- QuoteItemCoatDto: CatalogItemId + AddAsIncoming fields so the wizard can signal server-side
  incoming-item creation during quote save
- Inventory Create/Edit/Index views: IsIncoming badge and field
- IInventoryAiLookupService: minor interface update
- Migration: AddInventoryIsIncoming

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

src/PowderCoating.Core/Entities/InventoryItem.cs

@@ -58,6 +58,13 @@ public class InventoryItem : BaseEntity
     public bool IsActive { get; set; } = true;
     public DateTime? DiscontinuedDate { get; set; }
 
+    /// <summary>
+    /// True when this item was added to inventory as an ordered-but-not-yet-received powder.
+    /// Staff can quote and print QR codes while the powder is in transit.
+    /// Cleared automatically when a Purchase receipt is posted or staff manually unchecks it.
+    /// </summary>
+    public bool IsIncoming { get; set; } = false;
+
     // ── Financial Account Mapping ──────────────────────────────────────────
 
     /// <summary>