refactor(builder): remove dead FeaturesGrid mediaType field (D1c)

FeatureItem.mediaType was never read anywhere -- the icon-vs-image choice
is (correctly) driven by feat.image truthiness, set via the AssetPicker in
FeaturesEditor. Neither the default items nor FeaturesEditor's "add item"
object ever set it. Remove the dead field; image-precedence render/export
behavior is unchanged.

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

craft/src/components/sections/FeaturesGrid.tsx

@@ -7,7 +7,6 @@ interface FeatureItem {
   title: string;
   description: string;
   icon: string;
-  mediaType?: 'icon' | 'image';   // default behaves as 'icon' when undefined
   image?: string;
   imageAlt?: string;
   buttonText?: string;