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/Screens/CustomizeScreen.swift

@@ -42,7 +42,7 @@ struct CustomizeScreen {
     // MARK: - Actions
 
     /// Select a button in a horizontal picker. Scrolls vertically to reveal
-    /// the section, then scrolls horizontally within the picker to find the button.
+    /// the section, then scrolls horizontally to find the button.
     private func selectHorizontalPickerButton(
         _ button: XCUIElement,
         file: StaticString = #filePath,
@@ -55,31 +55,23 @@ struct CustomizeScreen {
         }
 
         // Phase 1: Scroll settings page vertically to reveal the section
-        let mainScroll = app.scrollViews.firstMatch
         for _ in 0..<5 {
             if button.exists && button.isHittable {
                 button.forceTap(file: file, line: line)
                 return
             }
-            mainScroll.swipeUp()
+            app.swipeUp()
         }
 
-        // Phase 2: Button is in hierarchy but off-screen in horizontal scroll.
-        // Find the horizontal scroll view containing the button and swipe within it.
+        // Phase 2: Button exists in tree but is off-screen in a horizontal ScrollView.
+        // Simple left swipes on the app to scroll horizontally.
         if button.exists {
-            // Swipe left on the button's parent region to scroll the horizontal picker
             for _ in 0..<8 {
                 if button.isHittable {
                     button.forceTap(file: file, line: line)
                     return
                 }
-                // Swipe left at the button's Y position to scroll the horizontal picker
-                let buttonFrame = button.frame
-                let startPoint = app.coordinate(withNormalizedOffset: CGVector(dx: 0.8, dy: 0))
-                    .withOffset(CGVector(dx: 0, dy: buttonFrame.midY))
-                let endPoint = app.coordinate(withNormalizedOffset: CGVector(dx: 0.2, dy: 0))
-                    .withOffset(CGVector(dx: 0, dy: buttonFrame.midY))
-                startPoint.press(forDuration: 0.05, thenDragTo: endPoint)
+                app.swipeLeft()
             }
         }
 
@@ -89,7 +81,7 @@ struct CustomizeScreen {
                 button.forceTap(file: file, line: line)
                 return
             }
-            mainScroll.swipeRight()
+            app.swipeRight()
         }
 
         XCTFail("Could not find or tap button: \(button)", file: file, line: line)