fix(build): resolve Xcode 26 compilation errors

- Add ServerConfigView stub (referenced in SilverAppleApp.swift)
- Add HardeningListView stub (referenced in DashboardView.swift)
- Fix .accentColor → Color.accentColor (ShapeStyle removed member)
- Fix withCheckedThrowingContinuation explicit type annotation
- Make AppEnvironment.serverUrl internal for ServerConfigView access

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

SilverApple/Onboarding/Views/MDMEnrollmentView.swift

@@ -15,7 +15,7 @@ struct MDMEnrollmentView: View {
             VStack(spacing: 32) {
                 Image(systemName: "iphone.badge.play")
                     .font(.system(size: 56))
-                    .foregroundStyle(.accentColor)
+                    .foregroundStyle(Color.accentColor)
 
                 VStack(spacing: 12) {
                     Text("Device Management")