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/OnboardingStyle.swift

@@ -9,6 +9,7 @@ import SwiftUI
 
 struct OnboardingStyle: View {
     @ObservedObject var onboardingData: OnboardingData
+    var onNext: (() -> Void)? = nil
     @State private var selectedTheme: AppTheme = .celestial
 
     var body: some View {
@@ -65,6 +66,22 @@ struct OnboardingStyle: View {
                     }
                     .padding(.horizontal, 20)
 
+                    // Continue button
+                    Button(action: { onNext?() }) {
+                        Text("Continue")
+                            .font(.headline.weight(.semibold))
+                            .foregroundColor(.white)
+                            .frame(maxWidth: .infinity)
+                            .padding(.vertical, 16)
+                            .background(
+                                RoundedRectangle(cornerRadius: 16)
+                                    .fill(.white.opacity(0.25))
+                            )
+                    }
+                    .padding(.horizontal, 20)
+                    .padding(.top, 24)
+                    .accessibilityIdentifier(AccessibilityID.Onboarding.nextButton)
+
                     // Hint
                     HStack(spacing: 8) {
                         Image(systemName: "arrow.left.arrow.right")
@@ -74,8 +91,8 @@ struct OnboardingStyle: View {
                             .fixedSize(horizontal: false, vertical: true)
                     }
                     .foregroundColor(.white.opacity(0.7))
-                    .padding(.top, 24)
-                    .padding(.bottom, 80)
+                    .padding(.top, 12)
+                    .padding(.bottom, 40)
                 }
         }
         .background(
@@ -91,6 +108,7 @@ struct OnboardingStyle: View {
             // Apply default theme on appear
             selectedTheme.apply()
         }
+        .accessibilityElement(children: .contain)
         .accessibilityIdentifier(AccessibilityID.Onboarding.styleScreen)
     }
 }