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/OnboardingDay.swift

@@ -23,6 +23,7 @@ enum DayOptions: Int, CaseIterable, RawRepresentable, Codable {
 
 struct OnboardingDay: View {
     @ObservedObject var onboardingData: OnboardingData
+    var onNext: (() -> Void)? = nil
 
     var body: some View {
         VStack(spacing: 0) {
@@ -89,6 +90,22 @@ struct OnboardingDay: View {
 
                 Spacer()
 
+                // Continue button
+                Button(action: { onNext?() }) {
+                    Text("Continue")
+                        .font(.headline.weight(.semibold))
+                        .foregroundColor(Color(hex: "4facfe"))
+                        .frame(maxWidth: .infinity)
+                        .padding(.vertical, 16)
+                        .background(
+                            RoundedRectangle(cornerRadius: 16)
+                                .fill(.white)
+                        )
+                }
+                .padding(.horizontal, 30)
+                .padding(.bottom, 30)
+                .accessibilityIdentifier(AccessibilityID.Onboarding.nextButton)
+
                 // Tip
                 HStack(spacing: 12) {
                     Image(systemName: "lightbulb.fill")
@@ -101,7 +118,7 @@ struct OnboardingDay: View {
                         .fixedSize(horizontal: false, vertical: true)
                 }
                 .padding(.horizontal, 30)
-                .padding(.bottom, 80)
+                .padding(.bottom, 40)
         }
         .background(
             LinearGradient(
@@ -111,6 +128,7 @@ struct OnboardingDay: View {
             )
             .ignoresSafeArea()
         )
+        .accessibilityElement(children: .contain)
         .accessibilityIdentifier(AccessibilityID.Onboarding.dayScreen)
     }