Warn on label scan when product already exists in tenant inventory

After resolving manufacturer + SKU from the scan, ScanLabel now queries the
tenant's InventoryItems: first by ManufacturerPartNumber exact match (most
precise), then by ColorName + Manufacturer fuzzy match as fallback.

If a match is found, the response includes existingInventoryId and
existingInventoryName. The JS fillFromScan() shows a warning banner with a
direct link to the existing item instead of the normal success message. Form
fields are still pre-filled so the user can proceed to add a new entry (e.g.
a different lot or bag size) if that was the intent.

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

src/PowderCoating.Web/wwwroot/js/inventory-label-scan.js

@@ -404,7 +404,16 @@
                 ? ' <span class="badge bg-success ms-1">Added to platform catalog</span>'
                 : '';
 
-        if (filled.length > 0) {
+        if (data.existingInventoryId) {
+            const itemName = data.existingInventoryName || data.colorName || 'This product';
+            const filledNote = filled.length > 0 ? ` Fields pre-filled from scan.` : '';
+            showFormStatus('warning',
+                `<i class="bi bi-exclamation-triangle-fill me-1"></i>` +
+                `<strong>${itemName}</strong> is already in your inventory. ` +
+                `<a href="/Inventory/Details/${data.existingInventoryId}" class="alert-link fw-semibold">View existing item</a>` +
+                ` &mdash; or continue below to add a new entry (e.g. a new lot or bag size).${filledNote}${catalogNote}`
+            );
+        } else if (filled.length > 0) {
             showFormStatus('success', `Filled from label scan: ${filled.join(', ')}.${catalogNote}`);
         } else {
             showFormStatus('warning', `Label scanned but no empty fields to fill.${catalogNote}`);