Fix QR detection (parallel loops), price extraction, and camera pre-warm

QR scanning:
- Run BarcodeDetector and jsQR in parallel — jsQR starts after JSQR_DELAY_MS
  (1.5 s) so both decode simultaneously. BarcodeDetector silently returns empty
  arrays for some QR variants; running jsQR in parallel via a separate rAF loop
  (rafId2) and its own off-screen canvas catches those cases. First decoder to
  find anything calls handleQrResult and sets qrFound = true; the other stops.

Price extraction (two bugs):
- ScanLabel: unitPrice was catalogMatch?.UnitPrice ?? 0m, ignoring aiResult
  .UnitCostPerLb entirely when no catalog match — changed to fall through to AI result
- AppendOffer: only read JSON-LD "price" field; Shopify AggregateOffer uses
  "lowPrice" instead — now checked as fallback so Prismatic Powders prices are found

Camera pre-warm:
- Reverted localStorage approach (caused getUserMedia to fire on every page load,
  showing Chrome's "Ask" prompt immediately before user clicked anything)
- Restored Permissions API gate: preWarmCamera only calls getUserMedia when
  navigator.permissions.query returns 'granted', never risks a page-load prompt

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

src/PowderCoating.Web/Controllers/InventoryController.cs

@@ -848,7 +848,7 @@ public class InventoryController : Controller
             requiresClearCoat     = catalogMatch?.RequiresClearCoat ?? aiResult.RequiresClearCoat,
             coverageSqFtPerLb     = catalogMatch?.CoverageSqFtPerLb ?? aiResult.CoverageSqFtPerLb,
             transferEfficiency    = catalogMatch?.TransferEfficiency ?? aiResult.TransferEfficiency,
-            unitPrice             = catalogMatch?.UnitPrice ?? 0m,
+            unitPrice             = catalogMatch?.UnitPrice ?? aiResult.UnitCostPerLb ?? 0m,
             imageUrl              = catalogMatch?.ImageUrl  ?? aiResult.ImageUrl,
             productUrl            = catalogMatch?.ProductUrl ?? aiResult.SpecPageUrl,
             sdsUrl                = catalogMatch?.SdsUrl ?? aiResult.SdsUrl,