Refactor ZStack layouts to .background(), add Year View accessibility IDs, triage QA test plan

Replace ZStack-with-gradient patterns with idiomatic .background() modifier
across onboarding, customize, and settings views. Add accessibility identifiers
to Year View charts for UI test automation. Mark 67 impossible-to-automate
tests RED in QA plan and scaffold initial Year View and Settings onboarding tests.

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

Shared/Onboarding/views/OnboardingDay.swift

@@ -25,16 +25,7 @@ struct OnboardingDay: View {
     @ObservedObject var onboardingData: OnboardingData
 
     var body: some View {
-        ZStack {
-            // Gradient background
-            LinearGradient(
-                colors: [Color(hex: "4facfe"), Color(hex: "00f2fe")],
-                startPoint: .topLeading,
-                endPoint: .bottomTrailing
-            )
-            .ignoresSafeArea()
-
-            VStack(spacing: 0) {
+        VStack(spacing: 0) {
                 Spacer()
 
                 // Icon
@@ -103,8 +94,15 @@ struct OnboardingDay: View {
                 }
                 .padding(.horizontal, 30)
                 .padding(.bottom, 80)
-            }
         }
+        .background(
+            LinearGradient(
+                colors: [Color(hex: "4facfe"), Color(hex: "00f2fe")],
+                startPoint: .topLeading,
+                endPoint: .bottomTrailing
+            )
+            .ignoresSafeArea()
+        )
         .accessibilityIdentifier(AccessibilityID.Onboarding.dayScreen)
     }
 }