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/OnboardingStyle.swift

@@ -12,17 +12,7 @@ struct OnboardingStyle: View {
     @State private var selectedTheme: AppTheme = .celestial
 
     var body: some View {
-        ZStack {
-            // Gradient background that updates with theme
-            LinearGradient(
-                colors: selectedTheme.previewColors,
-                startPoint: .topLeading,
-                endPoint: .bottomTrailing
-            )
-            .ignoresSafeArea()
-            .animation(.easeInOut(duration: 0.4), value: selectedTheme)
-
-            ScrollView(showsIndicators: false) {
+        ScrollView(showsIndicators: false) {
                 VStack(spacing: 0) {
                     // Icon
                     ZStack {
@@ -85,8 +75,16 @@ struct OnboardingStyle: View {
                     .padding(.top, 24)
                     .padding(.bottom, 80)
                 }
-            }
         }
+        .background(
+            LinearGradient(
+                colors: selectedTheme.previewColors,
+                startPoint: .topLeading,
+                endPoint: .bottomTrailing
+            )
+            .ignoresSafeArea()
+            .animation(.easeInOut(duration: 0.4), value: selectedTheme)
+        )
         .onAppear {
             // Apply default theme on appear
             selectedTheme.apply()