Add onboarding Next buttons and fix accessibility for paged TabView

App-side changes:
- Added "Get Started" / "Continue" next buttons to all onboarding pages
  (Welcome, Day, Time, Style) with onboarding_next_button accessibility ID
- Added onNext callback plumbing from OnboardingMain to each page
- OnboardingMain now uses TabView(selection:) for programmatic page navigation
- Added .accessibilityElement(children: .contain) to all onboarding pages
  to fix iOS 26 paged TabView not exposing child elements
- Added settings_segmented_picker accessibility ID to Settings Picker
- Reduced padding on onboarding pages to keep buttons in visible area

Test-side changes:
- OnboardingScreen: replaced unreliable swipeToNext() with tapNext()
  that taps the accessibility-identified next button
- OnboardingScreen: multi-strategy skip button detection for subscription page
- SettingsScreen: scoped segment tap to picker element to avoid tab bar collision
- CustomizeScreen: simplified horizontal scroll to plain app.swipeLeft()
- OnboardingVotingTests: uses tapNext() to advance to Day page

Passing: OnboardingTests.CompleteFlow, OnboardingVotingTests
Remaining: OnboardingTests.DoesNotRepeat (session state issue),
  Settings scroll (deep elements), Customize horizontal pickers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Shared/Onboarding/views/OnboardingMain.swift

@@ -11,22 +11,27 @@ struct OnboardingMain: View {
     @Environment(\.presentationMode) var presentationMode
     @State var onboardingData: OnboardingData
     @EnvironmentObject var iapManager: IAPManager
+    @State private var currentPage: Int = 0
 
     let updateBoardingDataClosure: ((OnboardingData) -> Void)
 
     var body: some View {
-        TabView {
+        TabView(selection: $currentPage) {
             // 1. Welcome screen
-            OnboardingWelcome()
+            OnboardingWelcome(onNext: nextPage)
+                .tag(0)
 
             // 2. Which day to rate
-            OnboardingDay(onboardingData: onboardingData)
+            OnboardingDay(onboardingData: onboardingData, onNext: nextPage)
+                .tag(1)
 
             // 3. Reminder time
-            OnboardingTime(onboardingData: onboardingData)
+            OnboardingTime(onboardingData: onboardingData, onNext: nextPage)
+                .tag(2)
 
             // 4. Style customization
-            OnboardingStyle(onboardingData: onboardingData)
+            OnboardingStyle(onboardingData: onboardingData, onNext: nextPage)
+                .tag(3)
 
             // 5. Subscription benefits & completion
             OnboardingSubscription(
@@ -35,6 +40,7 @@ struct OnboardingMain: View {
                     updateBoardingDataClosure(data)
                 }
             )
+            .tag(4)
         }
         .ignoresSafeArea()
         .tabViewStyle(.page)
@@ -44,6 +50,12 @@ struct OnboardingMain: View {
         .interactiveDismissDisabled()
     }
 
+    private func nextPage() {
+        withAnimation {
+            currentPage += 1
+        }
+    }
+
     func setupAppearance() {
         UIPageControl.appearance().currentPageIndicatorTintColor = .white
         UIPageControl.appearance().pageIndicatorTintColor = UIColor.white.withAlphaComponent(0.3)