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>

Tests iOS/OnboardingVotingTests.swift

@@ -16,8 +16,8 @@ final class OnboardingVotingTests: BaseUITestCase {
         let onboarding = OnboardingScreen(app: app)
         onboarding.assertVisible()
 
-        // Swipe from Welcome to the Day page
-        onboarding.swipeToNext()
+        // Advance from Welcome to Day page
+        onboarding.tapNext()
 
         // Tap Yesterday via accessibility ID
         let yesterdayButton = app.element(UITestID.Onboarding.dayYesterday)