feat(mobile-oidc): Story 3.1 AppLifecycle Token Validation

Summary

• Add WidgetsBindingObserver to AuthNotifier for app lifecycle token validation
• When app resumes from background, silently validate tokens and refresh if expired
• Guard against non-authenticated state to avoid spurious refresh attempts
• Code review fixes: ref.mounted guard in lifecycle callback, accurate log message for null expiration

Test plan

• 32/32 unit tests pass (flutter test)
• 11 new tests covering all 7 ACs (resume trigger, valid token, expired+refresh, both expired, observer add/remove, non-auth no-op, idempotence)
• Zero new lint issues (flutter analyze)
• Manual verification: background app >5min, resume → session validated or login screen shown