Implement screen reader detection service with reactive stream

Use a StreamController.broadcast() internally. Implement didChangeAccessibilityFeatures() from WidgetsBindingObserver — this is called on both iOS and Android when accessibility settings change. Supplement with didChangeAppLifecycleState() to catch cases where the user toggles accessibility while the app is backgrounded. To read the current state: use WidgetsBinding.instance.accessibilityFeatures.accessibleNavigation (available without a BuildContext).

Note MediaQuery.accessibleNavigation requires a BuildContext and cannot be used outside of widget tree — prefer WidgetsBinding.instance.accessibilityFeatures for service-layer detection. Integrate with AccessibilitySettingsRepository to read the screenReaderOverride flag — if override is true, emit true unconditionally. Deduplicate stream emissions to avoid causing redundant UI rebuilds: compare new value with last emitted before adding to the controller.

Write unit tests using flutter_test: (1) stream emits false by default (no screen reader active in test environment), (2) manual override forces stream to emit true, (3) didChangeAccessibilityFeatures triggers re-evaluation, (4) didChangeAppLifecycleState(resumed) triggers re-evaluation, (5) dispose removes observer and closes stream without error, (6) stream does not emit duplicate consecutive equal values. Use TestWidgetsFlutterBinding and mockMediaQuery to simulate accessibility state. Integration test on a device/simulator should verify real VoiceOver/TalkBack detection — tag as @Tags(['integration']). Minimum 80% branch coverage on the service.