Harden iOS app with audit fixes, UI consistency, and sheet race condition fixes

Applies verified fixes from deep audit (concurrency, performance, security,
accessibility), standardizes CRUD form buttons to Add/Save pattern, removes
.drawingGroup() that broke search bar TextFields, and converts vulnerable
.sheet(isPresented:) + if-let patterns to safe presentation to prevent
blank white modals.

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

iosApp/iosApp/Onboarding/OnboardingCoordinator.swift

@@ -28,12 +28,10 @@ struct OnboardingCoordinator: View {
     }
 
     private func goBack(to step: OnboardingStep) {
+        isNavigatingBack = true
         withAnimation(.easeInOut(duration: 0.3)) {
-            isNavigatingBack = true
             onboardingState.currentStep = step
-        }
-        // Reset after animation completes
-        DispatchQueue.main.asyncAfter(deadline: .now() + 0.35) {
+        } completion: {
             isNavigatingBack = false
         }
     }