Fix keyboard dismiss semantics across iOS and Android

[thymikee]

Summary

Fix keyboard dismissal semantics so dismissing the keyboard does not navigate away from the current screen.

Current behavior

• iOS returns UNSUPPORTED_OPERATION.
• Android dismisses the keyboard by sending back keyevents, which can navigate backward in-app.

Proposed scope

• Define platform-correct keyboard dismiss behavior for iOS and Android.
• Avoid using generic back navigation as the primary dismissal mechanism.
• Document exact semantics in CLI help and docs.

Acceptance criteria

• keyboard dismiss works on iOS and Android when the keyboard is visible.
• Dismissing the keyboard does not trigger unexpected back navigation.
• Failure modes are explicit when native dismissal is unavailable.