Warranty phase 2: real Dell provider + settings UI + PC hero badge

- DellProvider: real Dell TechDirect lookup. OAuth2 via HTTP Basic auth,
  asset-entitlements under /PROD/sbil/eapi/v5 (the device.warranty path 404s
  for this account), map latest dated entitlement to service level + dates.
  Cache the token process-wide; Dell rate-limits the token endpoint and a
  fresh request per refresh trips a 401 cooldown. Verified against live Dell.
- Lenovo/HP stay config-shaped stubs.
- Settings: warranty_dell_* keys (category integrations); Dell Warranty Lookup
  block in System Settings > Integrations (enable + client id/secret masked +
  optional token/API URL overrides).
- WarrantyPanel takes optional pre-fetched items; PCDetail fetches once and
  feeds both the panel and a new hero warranty-status/end-date badge.
- tools/mock_dell.py for offline testing of the provider flow.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/warranty/api/routes.py

@@ -17,7 +17,7 @@ from shopdb.api import (
 )
 
 from ..models import Warranty, WarrantyAsset
-from ..services import get_provider, ProviderNotConfigured
+from ..services import get_provider, ProviderNotConfigured, WarrantyLookupError
 
 warranty_bp = Blueprint('warranty', __name__)
 
@@ -194,7 +194,7 @@ def refresh_warranty(warrantyid):
     provider = get_provider(warranty.provider)
     try:
         result = provider.lookup(warranty.servicetag, warranty.vendor)
-    except ProviderNotConfigured as exc:
+    except (ProviderNotConfigured, WarrantyLookupError) as exc:
         return error_response(ErrorCodes.VALIDATION_ERROR, str(exc), http_code=400)
     if not result:
         return error_response(ErrorCodes.VALIDATION_ERROR,