Fix cascading crash and remaining UI test failures

- Revert key-by-key UserDefaults iteration that removed system keys
  causing kAXErrorServerNotFound crashes; restore removePersistentDomain
  with explicit subscription key clearing
- Add .accessibilityElement(children: .contain) to UpgradeBannerView
  so subscribe button is discoverable by XCUITest
- Fix AllDayViewStylesTests to use coordinate-based tapping instead of
  button.isHittable/button.tap() for iOS 26 Liquid Glass compatibility
- Improve OnboardingTests with multiple swipe retries and label-based
  fallback for skip button

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Shared/Views/SettingsView/SettingsTabView.swift

@@ -148,6 +148,7 @@ struct UpgradeBannerView: View {
             RoundedRectangle(cornerRadius: 14)
                 .fill(colorScheme == .dark ? Color(.systemGray6) : Color(.systemGray6).opacity(0.5))
         )
+        .accessibilityElement(children: .contain)
         .accessibilityIdentifier(AccessibilityID.Settings.upgradeBanner)
     }
 }