Onboarding: show the app icon with iOS rounded corners

The welcome page's green-gradient circle + SF symbol is replaced by the
app icon itself (AppIconArtwork imageset mirroring AppIcon.png), clipped
to the standard iOS rounded-rect corner radius (21pt at 96pt — ~22%),
so onboarding matches the Home Screen icon.

FuelBoard/OnboardingView.swift

@@ -96,12 +96,14 @@ struct OnboardingView: View {
         VStack(spacing: 0) {
             Spacer()
             ZStack {
-                Circle()
-                    .fill(LinearGradient(colors: [.green, .blue], startPoint: .topLeading, endPoint: .bottomTrailing))
+                // The app icon itself, with the standard iOS rounded-corner
+                // treatment (~22% of the size), so onboarding matches the
+                // Home Screen.
+                Image("AppIconArtwork")
+                    .resizable()
+                    .scaledToFill()
                     .frame(width: 96, height: 96)
-                Image(systemName: "fuelpump.fill")
-                    .font(.system(size: 44, weight: .bold))
-                    .foregroundStyle(.white)
+                    .clipShape(RoundedRectangle(cornerRadius: 21, style: .continuous))
             }
             .padding(.bottom, 28)