Add platform powder catalog, catalog-first lookup, and label scanner

- Platform PowderCatalogItem table (IPlainRepository, no tenant filter) with
  full spec fields: cure temp/time, finish, color families, clear coat flag,
  coverage sq ft/lb, transfer efficiency, IsUserContributed
- Two EF migrations: AddPowderCatalogItem + AddPowderCatalogSpecFields
- PowderCatalogController (SuperAdminOnly): import from Prismatic JSON scrape,
  Lookup AJAX endpoint (catalog-first, ranked by SKU exact match), stats view
  with Tenant Contributed card
- Unified smart Lookup button on inventory Create/Edit: catalog hit fills all
  fields via catalogSnapshot pattern; AI augments cure/finish data from product
  URL if subscription enabled; catalog miss falls through to AI lookup
- In-browser label scanner (_LabelScanModal): getUserMedia live camera feed,
  jsQR auto-detects QR codes in rAF loop; "Scan Label Text" fallback sends
  captured frame to Claude vision via /Inventory/ScanLabel
- ScanLabel endpoint handles both QR URL path (LookupByUrlAsync) and vision
  path (ScanLabelAsync); auto-inserts unrecognized products as
  IsUserContributed=true; returns wasInCatalog/addedToCatalog flags

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

src/PowderCoating.Web/Views/Shared/_Layout.cshtml

@@ -1308,6 +1308,10 @@
                         <i class="bi bi-database-fill-gear"></i>
                         <span>Seed Data</span>
                     </a>
+                    <a asp-controller="PowderCatalog" asp-action="Index" class="nav-link">
+                        <i class="bi bi-palette2"></i>
+                        <span>Powder Catalog</span>
+                    </a>
                     <a asp-controller="ManufacturerLookupPatterns" asp-action="Index" class="nav-link">
                         <i class="bi bi-link-45deg"></i>
                         <span>Manufacturer Lookup Patterns</span>