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.Infrastructure/Services/InventoryAiLookupService.cs

@@ -933,7 +933,9 @@ Rules:
     /// </summary>
     private static void AppendOffer(JsonElement offer, StringBuilder sb)
     {
-        var price    = offer.TryGetProperty("price",         out var p) ? p.ToString()   : null;
+        // Accept "price" (Offer) or "lowPrice" (AggregateOffer — used by Shopify and others)
+        var price    = offer.TryGetProperty("price",         out var p)  ? p.ToString()  :
+                       offer.TryGetProperty("lowPrice",      out var lp) ? lp.ToString() : null;
         var currency = offer.TryGetProperty("priceCurrency", out var c) ? c.GetString()  : "USD";
         var unit     = offer.TryGetProperty("unitText",      out var u) ? u.GetString()  : null;
         var avail    = offer.TryGetProperty("availability",  out var a) ? a.GetString()  : null;