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/AccessibilityIdentifiers.swift

@@ -80,6 +80,7 @@ enum AccessibilityID {
     // MARK: - Settings
     enum Settings {
         static let header = "settings_header"
+        static let segmentedPicker = "settings_segmented_picker"
         static let customizeTab = "settings_tab_customize"
         static let settingsTab = "settings_tab_settings"
         static let upgradeBanner = "upgrade_banner"
@@ -170,6 +171,7 @@ enum AccessibilityID {
         static let subscriptionScreen = "onboarding_subscription"
         static let subscribeButton = "onboarding_subscribe_button"
         static let skipButton = "onboarding_skip_button"
+        static let nextButton = "onboarding_next_button"
     }
 
     // MARK: - Reports