Open QR Label in modal instead of a new browser tab

Adds an embed mode to the Label view (hides standalone nav controls) and
an iframe-based modal on Inventory Details. The modal footer Print button
calls contentWindow.print() so the print dialog opens without spawning a
new window.

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

src/PowderCoating.Web/Controllers/InventoryController.cs

@@ -1623,11 +1623,12 @@ public class InventoryController : Controller
     /// Renders a print-optimised label for the inventory item containing the QR code,
     /// item name, SKU, and colour. Designed to be printed directly from the browser.
     /// </summary>
-    public async Task<IActionResult> Label(int? id)
+    public async Task<IActionResult> Label(int? id, bool embed = false)
     {
         if (id == null) return NotFound();
         var item = await _unitOfWork.InventoryItems.GetByIdAsync(id.Value);
         if (item == null) return NotFound();
+        ViewBag.IsEmbed = embed;
         return View(_mapper.Map<InventoryItemDto>(item));
     }